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   1  =head1 NAME
   2  
   3  perl56delta - what's new for perl v5.6.0
   4  
   5  =head1 DESCRIPTION
   6  
   7  This document describes differences between the 5.005 release and the 5.6.0
   8  release.
   9  
  10  =head1 Core Enhancements
  11  
  12  =head2 Interpreter cloning, threads, and concurrency
  13  
  14  Perl 5.6.0 introduces the beginnings of support for running multiple
  15  interpreters concurrently in different threads.  In conjunction with
  16  the perl_clone() API call, which can be used to selectively duplicate
  17  the state of any given interpreter, it is possible to compile a
  18  piece of code once in an interpreter, clone that interpreter
  19  one or more times, and run all the resulting interpreters in distinct
  20  threads.
  21  
  22  On the Windows platform, this feature is used to emulate fork() at the
  23  interpreter level.  See L<perlfork> for details about that.
  24  
  25  This feature is still in evolution.  It is eventually meant to be used
  26  to selectively clone a subroutine and data reachable from that
  27  subroutine in a separate interpreter and run the cloned subroutine
  28  in a separate thread.  Since there is no shared data between the
  29  interpreters, little or no locking will be needed (unless parts of
  30  the symbol table are explicitly shared).  This is obviously intended
  31  to be an easy-to-use replacement for the existing threads support.
  32  
  33  Support for cloning interpreters and interpreter concurrency can be
  34  enabled using the -Dusethreads Configure option (see win32/Makefile for
  35  how to enable it on Windows.)  The resulting perl executable will be
  36  functionally identical to one that was built with -Dmultiplicity, but
  37  the perl_clone() API call will only be available in the former.
  38  
  39  -Dusethreads enables the cpp macro USE_ITHREADS by default, which in turn
  40  enables Perl source code changes that provide a clear separation between
  41  the op tree and the data it operates with.  The former is immutable, and
  42  can therefore be shared between an interpreter and all of its clones,
  43  while the latter is considered local to each interpreter, and is therefore
  44  copied for each clone.
  45  
  46  Note that building Perl with the -Dusemultiplicity Configure option
  47  is adequate if you wish to run multiple B<independent> interpreters
  48  concurrently in different threads.  -Dusethreads only provides the
  49  additional functionality of the perl_clone() API call and other
  50  support for running B<cloned> interpreters concurrently.
  51  
  52      NOTE: This is an experimental feature.  Implementation details are
  53      subject to change.
  54  
  55  =head2 Lexically scoped warning categories
  56  
  57  You can now control the granularity of warnings emitted by perl at a finer
  58  level using the C<use warnings> pragma.  L<warnings> and L<perllexwarn>
  59  have copious documentation on this feature.
  60  
  61  =head2 Unicode and UTF-8 support
  62  
  63  Perl now uses UTF-8 as its internal representation for character
  64  strings.  The C<utf8> and C<bytes> pragmas are used to control this support
  65  in the current lexical scope.  See L<perlunicode>, L<utf8> and L<bytes> for
  66  more information.
  67  
  68  This feature is expected to evolve quickly to support some form of I/O
  69  disciplines that can be used to specify the kind of input and output data
  70  (bytes or characters).  Until that happens, additional modules from CPAN
  71  will be needed to complete the toolkit for dealing with Unicode.
  72  
  73      NOTE: This should be considered an experimental feature.  Implementation
  74      details are subject to change.
  75  
  76  =head2 Support for interpolating named characters
  77  
  78  The new C<\N> escape interpolates named characters within strings.
  79  For example, C<"Hi! \N{WHITE SMILING FACE}"> evaluates to a string
  80  with a unicode smiley face at the end.
  81  
  82  =head2 "our" declarations
  83  
  84  An "our" declaration introduces a value that can be best understood
  85  as a lexically scoped symbolic alias to a global variable in the
  86  package that was current where the variable was declared.  This is
  87  mostly useful as an alternative to the C<vars> pragma, but also provides
  88  the opportunity to introduce typing and other attributes for such
  89  variables.  See L<perlfunc/our>.
  90  
  91  =head2 Support for strings represented as a vector of ordinals
  92  
  93  Literals of the form C<v1.2.3.4> are now parsed as a string composed
  94  of characters with the specified ordinals.  This is an alternative, more
  95  readable way to construct (possibly unicode) strings instead of
  96  interpolating characters, as in C<"\x{1}\x{2}\x{3}\x{4}">.  The leading
  97  C<v> may be omitted if there are more than two ordinals, so C<1.2.3> is
  98  parsed the same as C<v1.2.3>.
  99  
 100  Strings written in this form are also useful to represent version "numbers".
 101  It is easy to compare such version "numbers" (which are really just plain
 102  strings) using any of the usual string comparison operators C<eq>, C<ne>,
 103  C<lt>, C<gt>, etc., or perform bitwise string operations on them using C<|>,
 104  C<&>, etc.
 105  
 106  In conjunction with the new C<$^V> magic variable (which contains
 107  the perl version as a string), such literals can be used as a readable way
 108  to check if you're running a particular version of Perl:
 109  
 110      # this will parse in older versions of Perl also
 111      if ($^V and $^V gt v5.6.0) {
 112          # new features supported
 113      }
 114  
 115  C<require> and C<use> also have some special magic to support such
 116  literals, but this particular usage should be avoided because it leads to
 117  misleading error messages under versions of Perl which don't support vector
 118  strings.  Using a true version number will ensure correct behavior in all
 119  versions of Perl:
 120  
 121      require 5.006;    # run time check for v5.6
 122      use 5.006_001;    # compile time check for v5.6.1
 123  
 124  Also, C<sprintf> and C<printf> support the Perl-specific format flag C<%v>
 125  to print ordinals of characters in arbitrary strings:
 126  
 127      printf "v%vd", $^V;        # prints current version, such as "v5.5.650"
 128      printf "%*vX", ":", $addr;    # formats IPv6 address
 129      printf "%*vb", " ", $bits;    # displays bitstring
 130  
 131  See L<perldata/"Scalar value constructors"> for additional information.
 132  
 133  =head2 Improved Perl version numbering system
 134  
 135  Beginning with Perl version 5.6.0, the version number convention has been
 136  changed to a "dotted integer" scheme that is more commonly found in open
 137  source projects.
 138  
 139  Maintenance versions of v5.6.0 will be released as v5.6.1, v5.6.2 etc.
 140  The next development series following v5.6.0 will be numbered v5.7.x,
 141  beginning with v5.7.0, and the next major production release following
 142  v5.6.0 will be v5.8.0.
 143  
 144  The English module now sets $PERL_VERSION to $^V (a string value) rather
 145  than C<$]> (a numeric value).  (This is a potential incompatibility.
 146  Send us a report via perlbug if you are affected by this.)
 147  
 148  The v1.2.3 syntax is also now legal in Perl.
 149  See L<Support for strings represented as a vector of ordinals> for more on that.
 150  
 151  To cope with the new versioning system's use of at least three significant
 152  digits for each version component, the method used for incrementing the
 153  subversion number has also changed slightly.  We assume that versions older
 154  than v5.6.0 have been incrementing the subversion component in multiples of
 155  10.  Versions after v5.6.0 will increment them by 1.  Thus, using the new
 156  notation, 5.005_03 is the "same" as v5.5.30, and the first maintenance
 157  version following v5.6.0 will be v5.6.1 (which should be read as being
 158  equivalent to a floating point value of 5.006_001 in the older format,
 159  stored in C<$]>).
 160  
 161  =head2 New syntax for declaring subroutine attributes
 162  
 163  Formerly, if you wanted to mark a subroutine as being a method call or
 164  as requiring an automatic lock() when it is entered, you had to declare
 165  that with a C<use attrs> pragma in the body of the subroutine.
 166  That can now be accomplished with declaration syntax, like this:
 167  
 168      sub mymethod : locked method;
 169      ...
 170      sub mymethod : locked method {
 171      ...
 172      }
 173  
 174      sub othermethod :locked :method;
 175      ...
 176      sub othermethod :locked :method {
 177      ...
 178      }
 179  
 180  
 181  (Note how only the first C<:> is mandatory, and whitespace surrounding
 182  the C<:> is optional.)
 183  
 184  F<AutoSplit.pm> and F<SelfLoader.pm> have been updated to keep the attributes
 185  with the stubs they provide.  See L<attributes>.
 186  
 187  =head2 File and directory handles can be autovivified
 188  
 189  Similar to how constructs such as C<< $x->[0] >> autovivify a reference,
 190  handle constructors (open(), opendir(), pipe(), socketpair(), sysopen(),
 191  socket(), and accept()) now autovivify a file or directory handle
 192  if the handle passed to them is an uninitialized scalar variable.  This
 193  allows the constructs such as C<open(my $fh, ...)> and C<open(local $fh,...)>
 194  to be used to create filehandles that will conveniently be closed
 195  automatically when the scope ends, provided there are no other references
 196  to them.  This largely eliminates the need for typeglobs when opening
 197  filehandles that must be passed around, as in the following example:
 198  
 199      sub myopen {
 200          open my $fh, "@_"
 201           or die "Can't open '@_': $!";
 202      return $fh;
 203      }
 204  
 205      {
 206          my $f = myopen("</etc/motd");
 207      print <$f>;
 208      # $f implicitly closed here
 209      }
 210  
 211  =head2 open() with more than two arguments
 212  
 213  If open() is passed three arguments instead of two, the second argument
 214  is used as the mode and the third argument is taken to be the file name.
 215  This is primarily useful for protecting against unintended magic behavior
 216  of the traditional two-argument form.  See L<perlfunc/open>.
 217  
 218  =head2 64-bit support
 219  
 220  Any platform that has 64-bit integers either
 221  
 222      (1) natively as longs or ints
 223      (2) via special compiler flags
 224      (3) using long long or int64_t
 225  
 226  is able to use "quads" (64-bit integers) as follows:
 227  
 228  =over 4
 229  
 230  =item *
 231  
 232  constants (decimal, hexadecimal, octal, binary) in the code 
 233  
 234  =item *
 235  
 236  arguments to oct() and hex()
 237  
 238  =item *
 239  
 240  arguments to print(), printf() and sprintf() (flag prefixes ll, L, q)
 241  
 242  =item *
 243  
 244  printed as such
 245  
 246  =item *
 247  
 248  pack() and unpack() "q" and "Q" formats
 249  
 250  =item *
 251  
 252  in basic arithmetics: + - * / % (NOTE: operating close to the limits
 253  of the integer values may produce surprising results)
 254  
 255  =item *
 256  
 257  in bit arithmetics: & | ^ ~ << >> (NOTE: these used to be forced 
 258  to be 32 bits wide but now operate on the full native width.)
 259  
 260  =item *
 261  
 262  vec()
 263  
 264  =back
 265  
 266  Note that unless you have the case (a) you will have to configure
 267  and compile Perl using the -Duse64bitint Configure flag.
 268  
 269      NOTE: The Configure flags -Duselonglong and -Duse64bits have been
 270      deprecated.  Use -Duse64bitint instead.
 271  
 272  There are actually two modes of 64-bitness: the first one is achieved
 273  using Configure -Duse64bitint and the second one using Configure
 274  -Duse64bitall.  The difference is that the first one is minimal and
 275  the second one maximal.  The first works in more places than the second.
 276  
 277  The C<use64bitint> does only as much as is required to get 64-bit
 278  integers into Perl (this may mean, for example, using "long longs")
 279  while your memory may still be limited to 2 gigabytes (because your
 280  pointers could still be 32-bit).  Note that the name C<64bitint> does
 281  not imply that your C compiler will be using 64-bit C<int>s (it might,
 282  but it doesn't have to): the C<use64bitint> means that you will be
 283  able to have 64 bits wide scalar values.
 284  
 285  The C<use64bitall> goes all the way by attempting to switch also
 286  integers (if it can), longs (and pointers) to being 64-bit.  This may
 287  create an even more binary incompatible Perl than -Duse64bitint: the
 288  resulting executable may not run at all in a 32-bit box, or you may
 289  have to reboot/reconfigure/rebuild your operating system to be 64-bit
 290  aware.
 291  
 292  Natively 64-bit systems like Alpha and Cray need neither -Duse64bitint
 293  nor -Duse64bitall.
 294  
 295  Last but not least: note that due to Perl's habit of always using
 296  floating point numbers, the quads are still not true integers.
 297  When quads overflow their limits (0...18_446_744_073_709_551_615 unsigned,
 298  -9_223_372_036_854_775_808...9_223_372_036_854_775_807 signed), they
 299  are silently promoted to floating point numbers, after which they will
 300  start losing precision (in their lower digits).
 301  
 302      NOTE: 64-bit support is still experimental on most platforms.
 303      Existing support only covers the LP64 data model.  In particular, the
 304      LLP64 data model is not yet supported.  64-bit libraries and system
 305      APIs on many platforms have not stabilized--your mileage may vary.
 306  
 307  =head2 Large file support
 308  
 309  If you have filesystems that support "large files" (files larger than
 310  2 gigabytes), you may now also be able to create and access them from
 311  Perl.
 312  
 313      NOTE: The default action is to enable large file support, if
 314      available on the platform.
 315  
 316  If the large file support is on, and you have a Fcntl constant
 317  O_LARGEFILE, the O_LARGEFILE is automatically added to the flags
 318  of sysopen().
 319  
 320  Beware that unless your filesystem also supports "sparse files" seeking
 321  to umpteen petabytes may be inadvisable.
 322  
 323  Note that in addition to requiring a proper file system to do large
 324  files you may also need to adjust your per-process (or your
 325  per-system, or per-process-group, or per-user-group) maximum filesize
 326  limits before running Perl scripts that try to handle large files,
 327  especially if you intend to write such files.
 328  
 329  Finally, in addition to your process/process group maximum filesize
 330  limits, you may have quota limits on your filesystems that stop you
 331  (your user id or your user group id) from using large files.
 332  
 333  Adjusting your process/user/group/file system/operating system limits
 334  is outside the scope of Perl core language.  For process limits, you
 335  may try increasing the limits using your shell's limits/limit/ulimit
 336  command before running Perl.  The BSD::Resource extension (not
 337  included with the standard Perl distribution) may also be of use, it
 338  offers the getrlimit/setrlimit interface that can be used to adjust
 339  process resource usage limits, including the maximum filesize limit.
 340  
 341  =head2 Long doubles
 342  
 343  In some systems you may be able to use long doubles to enhance the
 344  range and precision of your double precision floating point numbers
 345  (that is, Perl's numbers).  Use Configure -Duselongdouble to enable
 346  this support (if it is available).
 347  
 348  =head2 "more bits"
 349  
 350  You can "Configure -Dusemorebits" to turn on both the 64-bit support
 351  and the long double support.
 352  
 353  =head2 Enhanced support for sort() subroutines
 354  
 355  Perl subroutines with a prototype of C<($$)>, and XSUBs in general, can
 356  now be used as sort subroutines.  In either case, the two elements to
 357  be compared are passed as normal parameters in @_.  See L<perlfunc/sort>.
 358  
 359  For unprototyped sort subroutines, the historical behavior of passing 
 360  the elements to be compared as the global variables $a and $b remains
 361  unchanged.
 362  
 363  =head2 C<sort $coderef @foo> allowed
 364  
 365  sort() did not accept a subroutine reference as the comparison
 366  function in earlier versions.  This is now permitted.
 367  
 368  =head2 File globbing implemented internally
 369  
 370  Perl now uses the File::Glob implementation of the glob() operator
 371  automatically.  This avoids using an external csh process and the
 372  problems associated with it.
 373  
 374      NOTE: This is currently an experimental feature.  Interfaces and
 375      implementation are subject to change.
 376  
 377  =head2 Support for CHECK blocks
 378  
 379  In addition to C<BEGIN>, C<INIT>, C<END>, C<DESTROY> and C<AUTOLOAD>,
 380  subroutines named C<CHECK> are now special.  These are queued up during
 381  compilation and behave similar to END blocks, except they are called at
 382  the end of compilation rather than at the end of execution.  They cannot
 383  be called directly.
 384  
 385  =head2 POSIX character class syntax [: :] supported
 386  
 387  For example to match alphabetic characters use /[[:alpha:]]/.
 388  See L<perlre> for details.
 389  
 390  =head2 Better pseudo-random number generator
 391  
 392  In 5.005_0x and earlier, perl's rand() function used the C library
 393  rand(3) function.  As of 5.005_52, Configure tests for drand48(),
 394  random(), and rand() (in that order) and picks the first one it finds.
 395  
 396  These changes should result in better random numbers from rand().
 397  
 398  =head2 Improved C<qw//> operator
 399  
 400  The C<qw//> operator is now evaluated at compile time into a true list
 401  instead of being replaced with a run time call to C<split()>.  This
 402  removes the confusing misbehaviour of C<qw//> in scalar context, which
 403  had inherited that behaviour from split().
 404  
 405  Thus:
 406  
 407      $foo = ($bar) = qw(a b c); print "$foo|$bar\n";
 408  
 409  now correctly prints "3|a", instead of "2|a".
 410  
 411  =head2 Better worst-case behavior of hashes
 412  
 413  Small changes in the hashing algorithm have been implemented in
 414  order to improve the distribution of lower order bits in the
 415  hashed value.  This is expected to yield better performance on
 416  keys that are repeated sequences.
 417  
 418  =head2 pack() format 'Z' supported
 419  
 420  The new format type 'Z' is useful for packing and unpacking null-terminated
 421  strings.  See L<perlfunc/"pack">.
 422  
 423  =head2 pack() format modifier '!' supported
 424  
 425  The new format type modifier '!' is useful for packing and unpacking
 426  native shorts, ints, and longs.  See L<perlfunc/"pack">.
 427  
 428  =head2 pack() and unpack() support counted strings
 429  
 430  The template character '/' can be used to specify a counted string
 431  type to be packed or unpacked.  See L<perlfunc/"pack">.
 432  
 433  =head2 Comments in pack() templates
 434  
 435  The '#' character in a template introduces a comment up to
 436  end of the line.  This facilitates documentation of pack()
 437  templates.
 438  
 439  =head2 Weak references
 440  
 441  In previous versions of Perl, you couldn't cache objects so as
 442  to allow them to be deleted if the last reference from outside 
 443  the cache is deleted.  The reference in the cache would hold a
 444  reference count on the object and the objects would never be
 445  destroyed.
 446  
 447  Another familiar problem is with circular references.  When an
 448  object references itself, its reference count would never go
 449  down to zero, and it would not get destroyed until the program
 450  is about to exit.
 451  
 452  Weak references solve this by allowing you to "weaken" any
 453  reference, that is, make it not count towards the reference count.
 454  When the last non-weak reference to an object is deleted, the object
 455  is destroyed and all the weak references to the object are
 456  automatically undef-ed.
 457  
 458  To use this feature, you need the Devel::WeakRef package from CPAN, which
 459  contains additional documentation.
 460  
 461      NOTE: This is an experimental feature.  Details are subject to change.  
 462  
 463  =head2 Binary numbers supported
 464  
 465  Binary numbers are now supported as literals, in s?printf formats, and
 466  C<oct()>:
 467  
 468      $answer = 0b101010;
 469      printf "The answer is: %b\n", oct("0b101010");
 470  
 471  =head2 Lvalue subroutines
 472  
 473  Subroutines can now return modifiable lvalues.
 474  See L<perlsub/"Lvalue subroutines">.
 475  
 476      NOTE: This is an experimental feature.  Details are subject to change.
 477  
 478  =head2 Some arrows may be omitted in calls through references
 479  
 480  Perl now allows the arrow to be omitted in many constructs
 481  involving subroutine calls through references.  For example,
 482  C<< $foo[10]->('foo') >> may now be written C<$foo[10]('foo')>.
 483  This is rather similar to how the arrow may be omitted from
 484  C<< $foo[10]->{'foo'} >>.  Note however, that the arrow is still
 485  required for C<< foo(10)->('bar') >>.
 486  
 487  =head2 Boolean assignment operators are legal lvalues
 488  
 489  Constructs such as C<($a ||= 2) += 1> are now allowed.
 490  
 491  =head2 exists() is supported on subroutine names
 492  
 493  The exists() builtin now works on subroutine names.  A subroutine
 494  is considered to exist if it has been declared (even if implicitly).
 495  See L<perlfunc/exists> for examples.
 496  
 497  =head2 exists() and delete() are supported on array elements
 498  
 499  The exists() and delete() builtins now work on simple arrays as well.
 500  The behavior is similar to that on hash elements.
 501  
 502  exists() can be used to check whether an array element has been
 503  initialized.  This avoids autovivifying array elements that don't exist.
 504  If the array is tied, the EXISTS() method in the corresponding tied
 505  package will be invoked.
 506  
 507  delete() may be used to remove an element from the array and return
 508  it.  The array element at that position returns to its uninitialized
 509  state, so that testing for the same element with exists() will return
 510  false.  If the element happens to be the one at the end, the size of
 511  the array also shrinks up to the highest element that tests true for
 512  exists(), or 0 if none such is found.  If the array is tied, the DELETE() 
 513  method in the corresponding tied package will be invoked.
 514  
 515  See L<perlfunc/exists> and L<perlfunc/delete> for examples.
 516  
 517  =head2 Pseudo-hashes work better
 518  
 519  Dereferencing some types of reference values in a pseudo-hash,
 520  such as C<< $ph->{foo}[1] >>, was accidentally disallowed.  This has
 521  been corrected.
 522  
 523  When applied to a pseudo-hash element, exists() now reports whether
 524  the specified value exists, not merely if the key is valid.
 525  
 526  delete() now works on pseudo-hashes.  When given a pseudo-hash element
 527  or slice it deletes the values corresponding to the keys (but not the keys
 528  themselves).  See L<perlref/"Pseudo-hashes: Using an array as a hash">.
 529  
 530  Pseudo-hash slices with constant keys are now optimized to array lookups
 531  at compile-time.
 532  
 533  List assignments to pseudo-hash slices are now supported.
 534  
 535  The C<fields> pragma now provides ways to create pseudo-hashes, via
 536  fields::new() and fields::phash().  See L<fields>.
 537  
 538      NOTE: The pseudo-hash data type continues to be experimental.
 539      Limiting oneself to the interface elements provided by the
 540      fields pragma will provide protection from any future changes.
 541  
 542  =head2 Automatic flushing of output buffers
 543  
 544  fork(), exec(), system(), qx//, and pipe open()s now flush buffers
 545  of all files opened for output when the operation was attempted.  This
 546  mostly eliminates confusing buffering mishaps suffered by users unaware
 547  of how Perl internally handles I/O.
 548  
 549  This is not supported on some platforms like Solaris where a suitably
 550  correct implementation of fflush(NULL) isn't available.
 551  
 552  =head2 Better diagnostics on meaningless filehandle operations
 553  
 554  Constructs such as C<< open(<FH>) >> and C<< close(<FH>) >>
 555  are compile time errors.  Attempting to read from filehandles that
 556  were opened only for writing will now produce warnings (just as
 557  writing to read-only filehandles does).
 558  
 559  =head2 Where possible, buffered data discarded from duped input filehandle
 560  
 561  C<< open(NEW, "<&OLD") >> now attempts to discard any data that
 562  was previously read and buffered in C<OLD> before duping the handle.
 563  On platforms where doing this is allowed, the next read operation
 564  on C<NEW> will return the same data as the corresponding operation
 565  on C<OLD>.  Formerly, it would have returned the data from the start
 566  of the following disk block instead.
 567  
 568  =head2 eof() has the same old magic as <>
 569  
 570  C<eof()> would return true if no attempt to read from C<< <> >> had
 571  yet been made.  C<eof()> has been changed to have a little magic of its
 572  own, it now opens the C<< <> >> files.
 573  
 574  =head2 binmode() can be used to set :crlf and :raw modes
 575  
 576  binmode() now accepts a second argument that specifies a discipline
 577  for the handle in question.  The two pseudo-disciplines ":raw" and
 578  ":crlf" are currently supported on DOS-derivative platforms.
 579  See L<perlfunc/"binmode"> and L<open>.
 580  
 581  =head2 C<-T> filetest recognizes UTF-8 encoded files as "text"
 582  
 583  The algorithm used for the C<-T> filetest has been enhanced to
 584  correctly identify UTF-8 content as "text".
 585  
 586  =head2 system(), backticks and pipe open now reflect exec() failure
 587  
 588  On Unix and similar platforms, system(), qx() and open(FOO, "cmd |")
 589  etc., are implemented via fork() and exec().  When the underlying
 590  exec() fails, earlier versions did not report the error properly,
 591  since the exec() happened to be in a different process.
 592  
 593  The child process now communicates with the parent about the
 594  error in launching the external command, which allows these
 595  constructs to return with their usual error value and set $!.
 596  
 597  =head2 Improved diagnostics
 598  
 599  Line numbers are no longer suppressed (under most likely circumstances)
 600  during the global destruction phase.
 601  
 602  Diagnostics emitted from code running in threads other than the main
 603  thread are now accompanied by the thread ID.
 604  
 605  Embedded null characters in diagnostics now actually show up.  They
 606  used to truncate the message in prior versions.
 607  
 608  $foo::a and $foo::b are now exempt from "possible typo" warnings only
 609  if sort() is encountered in package C<foo>.
 610  
 611  Unrecognized alphabetic escapes encountered when parsing quote
 612  constructs now generate a warning, since they may take on new
 613  semantics in later versions of Perl.
 614  
 615  Many diagnostics now report the internal operation in which the warning
 616  was provoked, like so:
 617  
 618      Use of uninitialized value in concatenation (.) at (eval 1) line 1.
 619      Use of uninitialized value in print at (eval 1) line 1.
 620  
 621  Diagnostics  that occur within eval may also report the file and line
 622  number where the eval is located, in addition to the eval sequence
 623  number and the line number within the evaluated text itself.  For
 624  example:
 625  
 626      Not enough arguments for scalar at (eval 4)[newlib/perl5db.pl:1411] line 2, at EOF
 627  
 628  =head2 Diagnostics follow STDERR
 629  
 630  Diagnostic output now goes to whichever file the C<STDERR> handle
 631  is pointing at, instead of always going to the underlying C runtime
 632  library's C<stderr>.
 633  
 634  =head2 More consistent close-on-exec behavior
 635  
 636  On systems that support a close-on-exec flag on filehandles, the
 637  flag is now set for any handles created by pipe(), socketpair(),
 638  socket(), and accept(), if that is warranted by the value of $^F
 639  that may be in effect.  Earlier versions neglected to set the flag
 640  for handles created with these operators.  See L<perlfunc/pipe>,
 641  L<perlfunc/socketpair>, L<perlfunc/socket>, L<perlfunc/accept>,
 642  and L<perlvar/$^F>.
 643  
 644  =head2 syswrite() ease-of-use
 645  
 646  The length argument of C<syswrite()> has become optional.
 647  
 648  =head2 Better syntax checks on parenthesized unary operators
 649  
 650  Expressions such as:
 651  
 652      print defined(&foo,&bar,&baz);
 653      print uc("foo","bar","baz");
 654      undef($foo,&bar);
 655  
 656  used to be accidentally allowed in earlier versions, and produced
 657  unpredictable behaviour.  Some produced ancillary warnings
 658  when used in this way; others silently did the wrong thing.
 659  
 660  The parenthesized forms of most unary operators that expect a single
 661  argument now ensure that they are not called with more than one
 662  argument, making the cases shown above syntax errors.  The usual
 663  behaviour of:
 664  
 665      print defined &foo, &bar, &baz;
 666      print uc "foo", "bar", "baz";
 667      undef $foo, &bar;
 668  
 669  remains unchanged.  See L<perlop>.
 670  
 671  =head2 Bit operators support full native integer width
 672  
 673  The bit operators (& | ^ ~ << >>) now operate on the full native
 674  integral width (the exact size of which is available in $Config{ivsize}).
 675  For example, if your platform is either natively 64-bit or if Perl
 676  has been configured to use 64-bit integers, these operations apply
 677  to 8 bytes (as opposed to 4 bytes on 32-bit platforms).
 678  For portability, be sure to mask off the excess bits in the result of
 679  unary C<~>, e.g., C<~$x & 0xffffffff>.
 680  
 681  =head2 Improved security features
 682  
 683  More potentially unsafe operations taint their results for improved
 684  security.
 685  
 686  The C<passwd> and C<shell> fields returned by the getpwent(), getpwnam(),
 687  and getpwuid() are now tainted, because the user can affect their own
 688  encrypted password and login shell.
 689  
 690  The variable modified by shmread(), and messages returned by msgrcv()
 691  (and its object-oriented interface IPC::SysV::Msg::rcv) are also tainted,
 692  because other untrusted processes can modify messages and shared memory
 693  segments for their own nefarious purposes.
 694  
 695  =head2 More functional bareword prototype (*)
 696  
 697  Bareword prototypes have been rationalized to enable them to be used
 698  to override builtins that accept barewords and interpret them in
 699  a special way, such as C<require> or C<do>.
 700  
 701  Arguments prototyped as C<*> will now be visible within the subroutine
 702  as either a simple scalar or as a reference to a typeglob.
 703  See L<perlsub/Prototypes>.
 704  
 705  =head2 C<require> and C<do> may be overridden
 706  
 707  C<require> and C<do 'file'> operations may be overridden locally
 708  by importing subroutines of the same name into the current package 
 709  (or globally by importing them into the CORE::GLOBAL:: namespace).
 710  Overriding C<require> will also affect C<use>, provided the override
 711  is visible at compile-time.
 712  See L<perlsub/"Overriding Built-in Functions">.
 713  
 714  =head2 $^X variables may now have names longer than one character
 715  
 716  Formerly, $^X was synonymous with ${"\cX"}, but $^XY was a syntax
 717  error.  Now variable names that begin with a control character may be
 718  arbitrarily long.  However, for compatibility reasons, these variables
 719  I<must> be written with explicit braces, as C<${^XY}> for example.
 720  C<${^XYZ}> is synonymous with ${"\cXYZ"}.  Variable names with more
 721  than one control character, such as C<${^XY^Z}>, are illegal.
 722  
 723  The old syntax has not changed.  As before, `^X' may be either a
 724  literal control-X character or the two-character sequence `caret' plus
 725  `X'.  When braces are omitted, the variable name stops after the
 726  control character.  Thus C<"$^XYZ"> continues to be synonymous with
 727  C<$^X . "YZ"> as before.
 728  
 729  As before, lexical variables may not have names beginning with control
 730  characters.  As before, variables whose names begin with a control
 731  character are always forced to be in package `main'.  All such variables
 732  are reserved for future extensions, except those that begin with
 733  C<^_>, which may be used by user programs and are guaranteed not to
 734  acquire special meaning in any future version of Perl.
 735  
 736  =head2 New variable $^C reflects C<-c> switch
 737  
 738  C<$^C> has a boolean value that reflects whether perl is being run
 739  in compile-only mode (i.e. via the C<-c> switch).  Since
 740  BEGIN blocks are executed under such conditions, this variable
 741  enables perl code to determine whether actions that make sense
 742  only during normal running are warranted.  See L<perlvar>.
 743  
 744  =head2 New variable $^V contains Perl version as a string
 745  
 746  C<$^V> contains the Perl version number as a string composed of
 747  characters whose ordinals match the version numbers, i.e. v5.6.0.
 748  This may be used in string comparisons.
 749  
 750  See C<Support for strings represented as a vector of ordinals> for an
 751  example.
 752  
 753  =head2 Optional Y2K warnings
 754  
 755  If Perl is built with the cpp macro C<PERL_Y2KWARN> defined,
 756  it emits optional warnings when concatenating the number 19
 757  with another number.
 758  
 759  This behavior must be specifically enabled when running Configure.
 760  See F<INSTALL> and F<README.Y2K>.
 761  
 762  =head2 Arrays now always interpolate into double-quoted strings
 763  
 764  In double-quoted strings, arrays now interpolate, no matter what.  The
 765  behavior in earlier versions of perl 5 was that arrays would interpolate
 766  into strings if the array had been mentioned before the string was
 767  compiled, and otherwise Perl would raise a fatal compile-time error.
 768  In versions 5.000 through 5.003, the error was
 769  
 770          Literal @example now requires backslash
 771  
 772  In versions 5.004_01 through 5.6.0, the error was
 773  
 774          In string, @example now must be written as \@example
 775  
 776  The idea here was to get people into the habit of writing
 777  C<"fred\@example.com"> when they wanted a literal C<@> sign, just as
 778  they have always written C<"Give me back my \$5"> when they wanted a
 779  literal C<$> sign.
 780  
 781  Starting with 5.6.1, when Perl now sees an C<@> sign in a
 782  double-quoted string, it I<always> attempts to interpolate an array,
 783  regardless of whether or not the array has been used or declared
 784  already.  The fatal error has been downgraded to an optional warning:
 785  
 786          Possible unintended interpolation of @example in string
 787  
 788  This warns you that C<"fred@example.com"> is going to turn into
 789  C<fred.com> if you don't backslash the C<@>.
 790  See http://www.plover.com/~mjd/perl/at-error.html for more details
 791  about the history here.
 792  
 793  =head2 @- and @+ provide starting/ending offsets of regex matches
 794  
 795  The new magic variables @- and @+ provide the starting and ending
 796  offsets, respectively, of $&, $1, $2, etc.  See L<perlvar> for
 797  details.
 798  
 799  =head1 Modules and Pragmata
 800  
 801  =head2 Modules
 802  
 803  =over 4
 804  
 805  =item attributes
 806  
 807  While used internally by Perl as a pragma, this module also
 808  provides a way to fetch subroutine and variable attributes.
 809  See L<attributes>.
 810  
 811  =item B
 812  
 813  The Perl Compiler suite has been extensively reworked for this
 814  release.  More of the standard Perl test suite passes when run
 815  under the Compiler, but there is still a significant way to
 816  go to achieve production quality compiled executables.
 817  
 818      NOTE: The Compiler suite remains highly experimental.  The
 819      generated code may not be correct, even when it manages to execute
 820      without errors.
 821  
 822  =item Benchmark
 823  
 824  Overall, Benchmark results exhibit lower average error and better timing
 825  accuracy.  
 826  
 827  You can now run tests for I<n> seconds instead of guessing the right
 828  number of tests to run: e.g., timethese(-5, ...) will run each 
 829  code for at least 5 CPU seconds.  Zero as the "number of repetitions"
 830  means "for at least 3 CPU seconds".  The output format has also
 831  changed.  For example:
 832  
 833     use Benchmark;$x=3;timethese(-5,{a=>sub{$x*$x},b=>sub{$x**2}})
 834  
 835  will now output something like this:
 836  
 837     Benchmark: running a, b, each for at least 5 CPU seconds...
 838              a:  5 wallclock secs ( 5.77 usr +  0.00 sys =  5.77 CPU) @ 200551.91/s (n=1156516)
 839              b:  4 wallclock secs ( 5.00 usr +  0.02 sys =  5.02 CPU) @ 159605.18/s (n=800686)
 840  
 841  New features: "each for at least N CPU seconds...", "wallclock secs",
 842  and the "@ operations/CPU second (n=operations)".
 843  
 844  timethese() now returns a reference to a hash of Benchmark objects containing
 845  the test results, keyed on the names of the tests.
 846  
 847  timethis() now returns the iterations field in the Benchmark result object
 848  instead of 0.
 849  
 850  timethese(), timethis(), and the new cmpthese() (see below) can also take
 851  a format specifier of 'none' to suppress output.
 852  
 853  A new function countit() is just like timeit() except that it takes a
 854  TIME instead of a COUNT.
 855  
 856  A new function cmpthese() prints a chart comparing the results of each test
 857  returned from a timethese() call.  For each possible pair of tests, the
 858  percentage speed difference (iters/sec or seconds/iter) is shown.
 859  
 860  For other details, see L<Benchmark>.
 861  
 862  =item ByteLoader
 863  
 864  The ByteLoader is a dedicated extension to generate and run
 865  Perl bytecode.  See L<ByteLoader>.
 866  
 867  =item constant
 868  
 869  References can now be used.
 870  
 871  The new version also allows a leading underscore in constant names, but
 872  disallows a double leading underscore (as in "__LINE__").  Some other names
 873  are disallowed or warned against, including BEGIN, END, etc.  Some names
 874  which were forced into main:: used to fail silently in some cases; now they're
 875  fatal (outside of main::) and an optional warning (inside of main::).
 876  The ability to detect whether a constant had been set with a given name has
 877  been added.
 878  
 879  See L<constant>.
 880  
 881  =item charnames
 882  
 883  This pragma implements the C<\N> string escape.  See L<charnames>.
 884  
 885  =item Data::Dumper
 886  
 887  A C<Maxdepth> setting can be specified to avoid venturing
 888  too deeply into deep data structures.  See L<Data::Dumper>.
 889  
 890  The XSUB implementation of Dump() is now automatically called if the
 891  C<Useqq> setting is not in use.
 892  
 893  Dumping C<qr//> objects works correctly.
 894  
 895  =item DB
 896  
 897  C<DB> is an experimental module that exposes a clean abstraction
 898  to Perl's debugging API.
 899  
 900  =item DB_File
 901  
 902  DB_File can now be built with Berkeley DB versions 1, 2 or 3.
 903  See C<ext/DB_File/Changes>.
 904  
 905  =item Devel::DProf
 906  
 907  Devel::DProf, a Perl source code profiler has been added.  See
 908  L<Devel::DProf> and L<dprofpp>.
 909  
 910  =item Devel::Peek
 911  
 912  The Devel::Peek module provides access to the internal representation
 913  of Perl variables and data.  It is a data debugging tool for the XS programmer.
 914  
 915  =item Dumpvalue
 916  
 917  The Dumpvalue module provides screen dumps of Perl data.
 918  
 919  =item DynaLoader
 920  
 921  DynaLoader now supports a dl_unload_file() function on platforms that
 922  support unloading shared objects using dlclose().
 923  
 924  Perl can also optionally arrange to unload all extension shared objects
 925  loaded by Perl.  To enable this, build Perl with the Configure option
 926  C<-Accflags=-DDL_UNLOAD_ALL_AT_EXIT>.  (This maybe useful if you are
 927  using Apache with mod_perl.)
 928  
 929  =item English
 930  
 931  $PERL_VERSION now stands for C<$^V> (a string value) rather than for C<$]>
 932  (a numeric value).
 933  
 934  =item Env
 935  
 936  Env now supports accessing environment variables like PATH as array
 937  variables.
 938  
 939  =item Fcntl
 940  
 941  More Fcntl constants added: F_SETLK64, F_SETLKW64, O_LARGEFILE for
 942  large file (more than 4GB) access (NOTE: the O_LARGEFILE is
 943  automatically added to sysopen() flags if large file support has been
 944  configured, as is the default), Free/Net/OpenBSD locking behaviour
 945  flags F_FLOCK, F_POSIX, Linux F_SHLCK, and O_ACCMODE: the combined
 946  mask of O_RDONLY, O_WRONLY, and O_RDWR.  The seek()/sysseek()
 947  constants SEEK_SET, SEEK_CUR, and SEEK_END are available via the
 948  C<:seek> tag.  The chmod()/stat() S_IF* constants and S_IS* functions
 949  are available via the C<:mode> tag.
 950  
 951  =item File::Compare
 952  
 953  A compare_text() function has been added, which allows custom
 954  comparison functions.  See L<File::Compare>.
 955  
 956  =item File::Find
 957  
 958  File::Find now works correctly when the wanted() function is either
 959  autoloaded or is a symbolic reference.
 960  
 961  A bug that caused File::Find to lose track of the working directory
 962  when pruning top-level directories has been fixed.
 963  
 964  File::Find now also supports several other options to control its
 965  behavior.  It can follow symbolic links if the C<follow> option is
 966  specified.  Enabling the C<no_chdir> option will make File::Find skip
 967  changing the current directory when walking directories.  The C<untaint>
 968  flag can be useful when running with taint checks enabled.
 969  
 970  See L<File::Find>.
 971  
 972  =item File::Glob
 973  
 974  This extension implements BSD-style file globbing.  By default,
 975  it will also be used for the internal implementation of the glob()
 976  operator.  See L<File::Glob>.
 977  
 978  =item File::Spec
 979  
 980  New methods have been added to the File::Spec module: devnull() returns
 981  the name of the null device (/dev/null on Unix) and tmpdir() the name of
 982  the temp directory (normally /tmp on Unix).  There are now also methods
 983  to convert between absolute and relative filenames: abs2rel() and
 984  rel2abs().  For compatibility with operating systems that specify volume
 985  names in file paths, the splitpath(), splitdir(), and catdir() methods
 986  have been added.
 987  
 988  =item File::Spec::Functions
 989  
 990  The new File::Spec::Functions modules provides a function interface
 991  to the File::Spec module.  Allows shorthand
 992  
 993      $fullname = catfile($dir1, $dir2, $file);
 994  
 995  instead of
 996  
 997      $fullname = File::Spec->catfile($dir1, $dir2, $file);
 998  
 999  =item Getopt::Long
1000  
1001  Getopt::Long licensing has changed to allow the Perl Artistic License
1002  as well as the GPL. It used to be GPL only, which got in the way of
1003  non-GPL applications that wanted to use Getopt::Long.
1004  
1005  Getopt::Long encourages the use of Pod::Usage to produce help
1006  messages. For example:
1007  
1008      use Getopt::Long;
1009      use Pod::Usage;
1010      my $man = 0;
1011      my $help = 0;
1012      GetOptions('help|?' => \$help, man => \$man) or pod2usage(2);
1013      pod2usage(1) if $help;
1014      pod2usage(-exitstatus => 0, -verbose => 2) if $man;
1015  
1016      __END__
1017  
1018      =head1 NAME
1019  
1020      sample - Using Getopt::Long and Pod::Usage
1021  
1022      =head1 SYNOPSIS
1023  
1024      sample [options] [file ...]
1025  
1026       Options:
1027         -help            brief help message
1028         -man             full documentation
1029  
1030      =head1 OPTIONS
1031  
1032      =over 8
1033  
1034      =item B<-help>
1035  
1036      Print a brief help message and exits.
1037  
1038      =item B<-man>
1039  
1040      Prints the manual page and exits.
1041  
1042      =back
1043  
1044      =head1 DESCRIPTION
1045  
1046      B<This program> will read the given input file(s) and do something
1047      useful with the contents thereof.
1048  
1049      =cut
1050  
1051  See L<Pod::Usage> for details.
1052  
1053  A bug that prevented the non-option call-back <> from being
1054  specified as the first argument has been fixed.
1055  
1056  To specify the characters < and > as option starters, use ><. Note,
1057  however, that changing option starters is strongly deprecated. 
1058  
1059  =item IO
1060  
1061  write() and syswrite() will now accept a single-argument
1062  form of the call, for consistency with Perl's syswrite().
1063  
1064  You can now create a TCP-based IO::Socket::INET without forcing
1065  a connect attempt.  This allows you to configure its options
1066  (like making it non-blocking) and then call connect() manually.
1067  
1068  A bug that prevented the IO::Socket::protocol() accessor
1069  from ever returning the correct value has been corrected.
1070  
1071  IO::Socket::connect now uses non-blocking IO instead of alarm()
1072  to do connect timeouts.
1073  
1074  IO::Socket::accept now uses select() instead of alarm() for doing
1075  timeouts.
1076  
1077  IO::Socket::INET->new now sets $! correctly on failure. $@ is
1078  still set for backwards compatibility.
1079  
1080  =item JPL
1081  
1082  Java Perl Lingo is now distributed with Perl.  See jpl/README
1083  for more information.
1084  
1085  =item lib
1086  
1087  C<use lib> now weeds out any trailing duplicate entries.
1088  C<no lib> removes all named entries.
1089  
1090  =item Math::BigInt
1091  
1092  The bitwise operations C<<< << >>>, C<<< >> >>>, C<&>, C<|>,
1093  and C<~> are now supported on bigints.
1094  
1095  =item Math::Complex
1096  
1097  The accessor methods Re, Im, arg, abs, rho, and theta can now also
1098  act as mutators (accessor $z->Re(), mutator $z->Re(3)).
1099  
1100  The class method C<display_format> and the corresponding object method
1101  C<display_format>, in addition to accepting just one argument, now can
1102  also accept a parameter hash.  Recognized keys of a parameter hash are
1103  C<"style">, which corresponds to the old one parameter case, and two
1104  new parameters: C<"format">, which is a printf()-style format string
1105  (defaults usually to C<"%.15g">, you can revert to the default by
1106  setting the format string to C<undef>) used for both parts of a
1107  complex number, and C<"polar_pretty_print"> (defaults to true),
1108  which controls whether an attempt is made to try to recognize small
1109  multiples and rationals of pi (2pi, pi/2) at the argument (angle) of a
1110  polar complex number.
1111  
1112  The potentially disruptive change is that in list context both methods
1113  now I<return the parameter hash>, instead of only the value of the
1114  C<"style"> parameter.
1115  
1116  =item Math::Trig
1117  
1118  A little bit of radial trigonometry (cylindrical and spherical),
1119  radial coordinate conversions, and the great circle distance were added.
1120  
1121  =item Pod::Parser, Pod::InputObjects
1122  
1123  Pod::Parser is a base class for parsing and selecting sections of
1124  pod documentation from an input stream.  This module takes care of
1125  identifying pod paragraphs and commands in the input and hands off the
1126  parsed paragraphs and commands to user-defined methods which are free
1127  to interpret or translate them as they see fit.
1128  
1129  Pod::InputObjects defines some input objects needed by Pod::Parser, and
1130  for advanced users of Pod::Parser that need more about a command besides
1131  its name and text.
1132  
1133  As of release 5.6.0 of Perl, Pod::Parser is now the officially sanctioned
1134  "base parser code" recommended for use by all pod2xxx translators.
1135  Pod::Text (pod2text) and Pod::Man (pod2man) have already been converted
1136  to use Pod::Parser and efforts to convert Pod::HTML (pod2html) are already
1137  underway.  For any questions or comments about pod parsing and translating
1138  issues and utilities, please use the pod-people@perl.org mailing list.
1139  
1140  For further information, please see L<Pod::Parser> and L<Pod::InputObjects>.
1141  
1142  =item Pod::Checker, podchecker
1143  
1144  This utility checks pod files for correct syntax, according to
1145  L<perlpod>.  Obvious errors are flagged as such, while warnings are
1146  printed for mistakes that can be handled gracefully.  The checklist is
1147  not complete yet.  See L<Pod::Checker>.
1148  
1149  =item Pod::ParseUtils, Pod::Find
1150  
1151  These modules provide a set of gizmos that are useful mainly for pod
1152  translators.  L<Pod::Find|Pod::Find> traverses directory structures and
1153  returns found pod files, along with their canonical names (like
1154  C<File::Spec::Unix>).  L<Pod::ParseUtils|Pod::ParseUtils> contains
1155  B<Pod::List> (useful for storing pod list information), B<Pod::Hyperlink>
1156  (for parsing the contents of C<LE<lt>E<gt>> sequences) and B<Pod::Cache>
1157  (for caching information about pod files, e.g., link nodes).
1158  
1159  =item Pod::Select, podselect
1160  
1161  Pod::Select is a subclass of Pod::Parser which provides a function
1162  named "podselect()" to filter out user-specified sections of raw pod
1163  documentation from an input stream. podselect is a script that provides
1164  access to Pod::Select from other scripts to be used as a filter.
1165  See L<Pod::Select>.
1166  
1167  =item Pod::Usage, pod2usage
1168  
1169  Pod::Usage provides the function "pod2usage()" to print usage messages for
1170  a Perl script based on its embedded pod documentation.  The pod2usage()
1171  function is generally useful to all script authors since it lets them
1172  write and maintain a single source (the pods) for documentation, thus
1173  removing the need to create and maintain redundant usage message text
1174  consisting of information already in the pods.
1175  
1176  There is also a pod2usage script which can be used from other kinds of
1177  scripts to print usage messages from pods (even for non-Perl scripts
1178  with pods embedded in comments).
1179  
1180  For details and examples, please see L<Pod::Usage>.
1181  
1182  =item Pod::Text and Pod::Man
1183  
1184  Pod::Text has been rewritten to use Pod::Parser.  While pod2text() is
1185  still available for backwards compatibility, the module now has a new
1186  preferred interface.  See L<Pod::Text> for the details.  The new Pod::Text
1187  module is easily subclassed for tweaks to the output, and two such
1188  subclasses (Pod::Text::Termcap for man-page-style bold and underlining
1189  using termcap information, and Pod::Text::Color for markup with ANSI color
1190  sequences) are now standard.
1191  
1192  pod2man has been turned into a module, Pod::Man, which also uses
1193  Pod::Parser.  In the process, several outstanding bugs related to quotes
1194  in section headers, quoting of code escapes, and nested lists have been
1195  fixed.  pod2man is now a wrapper script around this module.
1196  
1197  =item SDBM_File
1198  
1199  An EXISTS method has been added to this module (and sdbm_exists() has
1200  been added to the underlying sdbm library), so one can now call exists
1201  on an SDBM_File tied hash and get the correct result, rather than a
1202  runtime error.
1203  
1204  A bug that may have caused data loss when more than one disk block
1205  happens to be read from the database in a single FETCH() has been
1206  fixed.
1207  
1208  =item Sys::Syslog
1209  
1210  Sys::Syslog now uses XSUBs to access facilities from syslog.h so it
1211  no longer requires syslog.ph to exist. 
1212  
1213  =item Sys::Hostname
1214  
1215  Sys::Hostname now uses XSUBs to call the C library's gethostname() or
1216  uname() if they exist.
1217  
1218  =item Term::ANSIColor
1219  
1220  Term::ANSIColor is a very simple module to provide easy and readable
1221  access to the ANSI color and highlighting escape sequences, supported by
1222  most ANSI terminal emulators.  It is now included standard.
1223  
1224  =item Time::Local
1225  
1226  The timelocal() and timegm() functions used to silently return bogus
1227  results when the date fell outside the machine's integer range.  They
1228  now consistently croak() if the date falls in an unsupported range.
1229  
1230  =item Win32
1231  
1232  The error return value in list context has been changed for all functions
1233  that return a list of values.  Previously these functions returned a list
1234  with a single element C<undef> if an error occurred.  Now these functions
1235  return the empty list in these situations.  This applies to the following
1236  functions:
1237  
1238      Win32::FsType
1239      Win32::GetOSVersion
1240  
1241  The remaining functions are unchanged and continue to return C<undef> on
1242  error even in list context.
1243  
1244  The Win32::SetLastError(ERROR) function has been added as a complement
1245  to the Win32::GetLastError() function.
1246  
1247  The new Win32::GetFullPathName(FILENAME) returns the full absolute
1248  pathname for FILENAME in scalar context.  In list context it returns
1249  a two-element list containing the fully qualified directory name and
1250  the filename.  See L<Win32>.
1251  
1252  =item XSLoader
1253  
1254  The XSLoader extension is a simpler alternative to DynaLoader.
1255  See L<XSLoader>.
1256  
1257  =item DBM Filters
1258  
1259  A new feature called "DBM Filters" has been added to all the
1260  DBM modules--DB_File, GDBM_File, NDBM_File, ODBM_File, and SDBM_File.
1261  DBM Filters add four new methods to each DBM module:
1262  
1263      filter_store_key
1264      filter_store_value
1265      filter_fetch_key
1266      filter_fetch_value
1267  
1268  These can be used to filter key-value pairs before the pairs are
1269  written to the database or just after they are read from the database.
1270  See L<perldbmfilter> for further information.
1271  
1272  =back
1273  
1274  =head2 Pragmata
1275  
1276  C<use attrs> is now obsolete, and is only provided for
1277  backward-compatibility.  It's been replaced by the C<sub : attributes>
1278  syntax.  See L<perlsub/"Subroutine Attributes"> and L<attributes>.
1279  
1280  Lexical warnings pragma, C<use warnings;>, to control optional warnings.
1281  See L<perllexwarn>.
1282  
1283  C<use filetest> to control the behaviour of filetests (C<-r> C<-w>
1284  ...).  Currently only one subpragma implemented, "use filetest
1285  'access';", that uses access(2) or equivalent to check permissions
1286  instead of using stat(2) as usual.  This matters in filesystems
1287  where there are ACLs (access control lists): the stat(2) might lie,
1288  but access(2) knows better.
1289  
1290  The C<open> pragma can be used to specify default disciplines for
1291  handle constructors (e.g. open()) and for qx//.  The two
1292  pseudo-disciplines C<:raw> and C<:crlf> are currently supported on
1293  DOS-derivative platforms (i.e. where binmode is not a no-op).
1294  See also L</"binmode() can be used to set :crlf and :raw modes">.
1295  
1296  =head1 Utility Changes
1297  
1298  =head2 dprofpp
1299  
1300  C<dprofpp> is used to display profile data generated using C<Devel::DProf>.
1301  See L<dprofpp>.
1302  
1303  =head2 find2perl
1304  
1305  The C<find2perl> utility now uses the enhanced features of the File::Find
1306  module.  The -depth and -follow options are supported.  Pod documentation
1307  is also included in the script.
1308  
1309  =head2 h2xs
1310  
1311  The C<h2xs> tool can now work in conjunction with C<C::Scan> (available
1312  from CPAN) to automatically parse real-life header files.  The C<-M>,
1313  C<-a>, C<-k>, and C<-o> options are new.
1314  
1315  =head2 perlcc
1316  
1317  C<perlcc> now supports the C and Bytecode backends.  By default,
1318  it generates output from the simple C backend rather than the
1319  optimized C backend.
1320  
1321  Support for non-Unix platforms has been improved.
1322  
1323  =head2 perldoc
1324  
1325  C<perldoc> has been reworked to avoid possible security holes.
1326  It will not by default let itself be run as the superuser, but you
1327  may still use the B<-U> switch to try to make it drop privileges
1328  first.
1329  
1330  =head2 The Perl Debugger
1331  
1332  Many bug fixes and enhancements were added to F<perl5db.pl>, the
1333  Perl debugger.  The help documentation was rearranged.  New commands
1334  include C<< < ? >>, C<< > ? >>, and C<< { ? >> to list out current
1335  actions, C<man I<docpage>> to run your doc viewer on some perl
1336  docset, and support for quoted options.  The help information was
1337  rearranged, and should be viewable once again if you're using B<less>
1338  as your pager.  A serious security hole was plugged--you should
1339  immediately remove all older versions of the Perl debugger as
1340  installed in previous releases, all the way back to perl3, from
1341  your system to avoid being bitten by this.
1342  
1343  =head1 Improved Documentation
1344  
1345  Many of the platform-specific README files are now part of the perl
1346  installation.  See L<perl> for the complete list.
1347  
1348  =over 4
1349  
1350  =item perlapi.pod
1351  
1352  The official list of public Perl API functions.
1353  
1354  =item perlboot.pod
1355  
1356  A tutorial for beginners on object-oriented Perl.
1357  
1358  =item perlcompile.pod
1359  
1360  An introduction to using the Perl Compiler suite.
1361  
1362  =item perldbmfilter.pod
1363  
1364  A howto document on using the DBM filter facility.
1365  
1366  =item perldebug.pod
1367  
1368  All material unrelated to running the Perl debugger, plus all
1369  low-level guts-like details that risked crushing the casual user
1370  of the debugger, have been relocated from the old manpage to the
1371  next entry below.
1372  
1373  =item perldebguts.pod
1374  
1375  This new manpage contains excessively low-level material not related
1376  to the Perl debugger, but slightly related to debugging Perl itself.
1377  It also contains some arcane internal details of how the debugging
1378  process works that may only be of interest to developers of Perl
1379  debuggers.
1380  
1381  =item perlfork.pod
1382  
1383  Notes on the fork() emulation currently available for the Windows platform.
1384  
1385  =item perlfilter.pod
1386  
1387  An introduction to writing Perl source filters.
1388  
1389  =item perlhack.pod
1390  
1391  Some guidelines for hacking the Perl source code.
1392  
1393  =item perlintern.pod
1394  
1395  A list of internal functions in the Perl source code.
1396  (List is currently empty.)
1397  
1398  =item perllexwarn.pod
1399  
1400  Introduction and reference information about lexically scoped
1401  warning categories.
1402  
1403  =item perlnumber.pod
1404  
1405  Detailed information about numbers as they are represented in Perl.
1406  
1407  =item perlopentut.pod
1408  
1409  A tutorial on using open() effectively.
1410  
1411  =item perlreftut.pod
1412  
1413  A tutorial that introduces the essentials of references.
1414  
1415  =item perltootc.pod
1416  
1417  A tutorial on managing class data for object modules.
1418  
1419  =item perltodo.pod
1420  
1421  Discussion of the most often wanted features that may someday be
1422  supported in Perl.
1423  
1424  =item perlunicode.pod
1425  
1426  An introduction to Unicode support features in Perl.
1427  
1428  =back
1429  
1430  =head1 Performance enhancements
1431  
1432  =head2 Simple sort() using { $a <=> $b } and the like are optimized
1433  
1434  Many common sort() operations using a simple inlined block are now
1435  optimized for faster performance.
1436  
1437  =head2 Optimized assignments to lexical variables
1438  
1439  Certain operations in the RHS of assignment statements have been
1440  optimized to directly set the lexical variable on the LHS,
1441  eliminating redundant copying overheads.
1442  
1443  =head2 Faster subroutine calls
1444  
1445  Minor changes in how subroutine calls are handled internally
1446  provide marginal improvements in performance.
1447  
1448  =head2 delete(), each(), values() and hash iteration are faster
1449  
1450  The hash values returned by delete(), each(), values() and hashes in a
1451  list context are the actual values in the hash, instead of copies.
1452  This results in significantly better performance, because it eliminates
1453  needless copying in most situations.
1454  
1455  =head1 Installation and Configuration Improvements
1456  
1457  =head2 -Dusethreads means something different
1458  
1459  The -Dusethreads flag now enables the experimental interpreter-based thread
1460  support by default.  To get the flavor of experimental threads that was in
1461  5.005 instead, you need to run Configure with "-Dusethreads -Duse5005threads".
1462  
1463  As of v5.6.0, interpreter-threads support is still lacking a way to
1464  create new threads from Perl (i.e., C<use Thread;> will not work with
1465  interpreter threads).  C<use Thread;> continues to be available when you
1466  specify the -Duse5005threads option to Configure, bugs and all.
1467  
1468      NOTE: Support for threads continues to be an experimental feature.
1469      Interfaces and implementation are subject to sudden and drastic changes.
1470  
1471  =head2 New Configure flags
1472  
1473  The following new flags may be enabled on the Configure command line
1474  by running Configure with C<-Dflag>.
1475  
1476      usemultiplicity
1477      usethreads useithreads    (new interpreter threads: no Perl API yet)
1478      usethreads use5005threads    (threads as they were in 5.005)
1479  
1480      use64bitint            (equal to now deprecated 'use64bits')
1481      use64bitall
1482  
1483      uselongdouble
1484      usemorebits
1485      uselargefiles
1486      usesocks            (only SOCKS v5 supported)
1487  
1488  =head2 Threadedness and 64-bitness now more daring
1489  
1490  The Configure options enabling the use of threads and the use of
1491  64-bitness are now more daring in the sense that they no more have an
1492  explicit list of operating systems of known threads/64-bit
1493  capabilities.  In other words: if your operating system has the
1494  necessary APIs and datatypes, you should be able just to go ahead and
1495  use them, for threads by Configure -Dusethreads, and for 64 bits
1496  either explicitly by Configure -Duse64bitint or implicitly if your
1497  system has 64-bit wide datatypes.  See also L<"64-bit support">.
1498  
1499  =head2 Long Doubles
1500  
1501  Some platforms have "long doubles", floating point numbers of even
1502  larger range than ordinary "doubles".  To enable using long doubles for
1503  Perl's scalars, use -Duselongdouble.
1504  
1505  =head2 -Dusemorebits
1506  
1507  You can enable both -Duse64bitint and -Duselongdouble with -Dusemorebits.
1508  See also L<"64-bit support">.
1509  
1510  =head2 -Duselargefiles
1511  
1512  Some platforms support system APIs that are capable of handling large files
1513  (typically, files larger than two gigabytes).  Perl will try to use these
1514  APIs if you ask for -Duselargefiles.
1515  
1516  See L<"Large file support"> for more information. 
1517  
1518  =head2 installusrbinperl
1519  
1520  You can use "Configure -Uinstallusrbinperl" which causes installperl
1521  to skip installing perl also as /usr/bin/perl.  This is useful if you
1522  prefer not to modify /usr/bin for some reason or another but harmful
1523  because many scripts assume to find Perl in /usr/bin/perl.
1524  
1525  =head2 SOCKS support
1526  
1527  You can use "Configure -Dusesocks" which causes Perl to probe
1528  for the SOCKS proxy protocol library (v5, not v4).  For more information
1529  on SOCKS, see:
1530  
1531      http://www.socks.nec.com/
1532  
1533  =head2 C<-A> flag
1534  
1535  You can "post-edit" the Configure variables using the Configure C<-A>
1536  switch.  The editing happens immediately after the platform specific
1537  hints files have been processed but before the actual configuration
1538  process starts.  Run C<Configure -h> to find out the full C<-A> syntax.
1539  
1540  =head2 Enhanced Installation Directories
1541  
1542  The installation structure has been enriched to improve the support
1543  for maintaining multiple versions of perl, to provide locations for
1544  vendor-supplied modules, scripts, and manpages, and to ease maintenance
1545  of locally-added modules, scripts, and manpages.  See the section on
1546  Installation Directories in the INSTALL file for complete details.
1547  For most users building and installing from source, the defaults should
1548  be fine.
1549  
1550  If you previously used C<Configure -Dsitelib> or C<-Dsitearch> to set
1551  special values for library directories, you might wish to consider using
1552  the new C<-Dsiteprefix> setting instead.  Also, if you wish to re-use a
1553  config.sh file from an earlier version of perl, you should be sure to
1554  check that Configure makes sensible choices for the new directories.
1555  See INSTALL for complete details.
1556  
1557  =head1 Platform specific changes
1558  
1559  =head2 Supported platforms
1560  
1561  =over 4
1562  
1563  =item *
1564  
1565  The Mach CThreads (NEXTSTEP, OPENSTEP) are now supported by the Thread
1566  extension.
1567  
1568  =item *
1569  
1570  GNU/Hurd is now supported.
1571  
1572  =item *
1573  
1574  Rhapsody/Darwin is now supported.
1575  
1576  =item *
1577  
1578  EPOC is now supported (on Psion 5).
1579  
1580  =item *
1581  
1582  The cygwin port (formerly cygwin32) has been greatly improved.
1583  
1584  =back
1585  
1586  =head2 DOS
1587  
1588  =over 4
1589  
1590  =item *
1591  
1592  Perl now works with djgpp 2.02 (and 2.03 alpha).
1593  
1594  =item *
1595  
1596  Environment variable names are not converted to uppercase any more.
1597  
1598  =item *
1599  
1600  Incorrect exit codes from backticks have been fixed.
1601  
1602  =item *
1603  
1604  This port continues to use its own builtin globbing (not File::Glob).
1605  
1606  =back
1607  
1608  =head2 OS390 (OpenEdition MVS)
1609  
1610  Support for this EBCDIC platform has not been renewed in this release.
1611  There are difficulties in reconciling Perl's standardization on UTF-8
1612  as its internal representation for characters with the EBCDIC character
1613  set, because the two are incompatible.
1614  
1615  It is unclear whether future versions will renew support for this
1616  platform, but the possibility exists.
1617  
1618  =head2 VMS
1619  
1620  Numerous revisions and extensions to configuration, build, testing, and
1621  installation process to accommodate core changes and VMS-specific options.
1622  
1623  Expand %ENV-handling code to allow runtime mapping to logical names,
1624  CLI symbols, and CRTL environ array.
1625  
1626  Extension of subprocess invocation code to accept filespecs as command
1627  "verbs".
1628  
1629  Add to Perl command line processing the ability to use default file types and
1630  to recognize Unix-style C<2E<gt>&1>.
1631  
1632  Expansion of File::Spec::VMS routines, and integration into ExtUtils::MM_VMS.
1633  
1634  Extension of ExtUtils::MM_VMS to handle complex extensions more flexibly.
1635  
1636  Barewords at start of Unix-syntax paths may be treated as text rather than
1637  only as logical names.
1638  
1639  Optional secure translation of several logical names used internally by Perl.
1640  
1641  Miscellaneous bugfixing and porting of new core code to VMS.
1642  
1643  Thanks are gladly extended to the many people who have contributed VMS
1644  patches, testing, and ideas.
1645  
1646  =head2 Win32
1647  
1648  Perl can now emulate fork() internally, using multiple interpreters running
1649  in different concurrent threads.  This support must be enabled at build
1650  time.  See L<perlfork> for detailed information.
1651  
1652  When given a pathname that consists only of a drivename, such as C<A:>,
1653  opendir() and stat() now use the current working directory for the drive
1654  rather than the drive root.
1655  
1656  The builtin XSUB functions in the Win32:: namespace are documented.  See
1657  L<Win32>.
1658  
1659  $^X now contains the full path name of the running executable.
1660  
1661  A Win32::GetLongPathName() function is provided to complement
1662  Win32::GetFullPathName() and Win32::GetShortPathName().  See L<Win32>.
1663  
1664  POSIX::uname() is supported.
1665  
1666  system(1,...) now returns true process IDs rather than process
1667  handles.  kill() accepts any real process id, rather than strictly
1668  return values from system(1,...).
1669  
1670  For better compatibility with Unix, C<kill(0, $pid)> can now be used to
1671  test whether a process exists.
1672  
1673  The C<Shell> module is supported.
1674  
1675  Better support for building Perl under command.com in Windows 95
1676  has been added.
1677  
1678  Scripts are read in binary mode by default to allow ByteLoader (and
1679  the filter mechanism in general) to work properly.  For compatibility,
1680  the DATA filehandle will be set to text mode if a carriage return is
1681  detected at the end of the line containing the __END__ or __DATA__
1682  token; if not, the DATA filehandle will be left open in binary mode.
1683  Earlier versions always opened the DATA filehandle in text mode.
1684  
1685  The glob() operator is implemented via the C<File::Glob> extension,
1686  which supports glob syntax of the C shell.  This increases the flexibility
1687  of the glob() operator, but there may be compatibility issues for
1688  programs that relied on the older globbing syntax.  If you want to
1689  preserve compatibility with the older syntax, you might want to run
1690  perl with C<-MFile::DosGlob>.  For details and compatibility information,
1691  see L<File::Glob>.
1692  
1693  =head1 Significant bug fixes
1694  
1695  =head2 <HANDLE> on empty files
1696  
1697  With C<$/> set to C<undef>, "slurping" an empty file returns a string of
1698  zero length (instead of C<undef>, as it used to) the first time the
1699  HANDLE is read after C<$/> is set to C<undef>.  Further reads yield
1700  C<undef>.
1701  
1702  This means that the following will append "foo" to an empty file (it used
1703  to do nothing):
1704  
1705      perl -0777 -pi -e 's/^/foo/' empty_file
1706  
1707  The behaviour of:
1708  
1709      perl -pi -e 's/^/foo/' empty_file
1710  
1711  is unchanged (it continues to leave the file empty).
1712  
1713  =head2 C<eval '...'> improvements
1714  
1715  Line numbers (as reflected by caller() and most diagnostics) within
1716  C<eval '...'> were often incorrect where here documents were involved.
1717  This has been corrected.
1718  
1719  Lexical lookups for variables appearing in C<eval '...'> within
1720  functions that were themselves called within an C<eval '...'> were
1721  searching the wrong place for lexicals.  The lexical search now
1722  correctly ends at the subroutine's block boundary.
1723  
1724  The use of C<return> within C<eval {...}> caused $@ not to be reset
1725  correctly when no exception occurred within the eval.  This has
1726  been fixed.
1727  
1728  Parsing of here documents used to be flawed when they appeared as
1729  the replacement expression in C<eval 's/.../.../e'>.  This has
1730  been fixed.
1731  
1732  =head2 All compilation errors are true errors
1733  
1734  Some "errors" encountered at compile time were by necessity 
1735  generated as warnings followed by eventual termination of the
1736  program.  This enabled more such errors to be reported in a
1737  single run, rather than causing a hard stop at the first error
1738  that was encountered.
1739  
1740  The mechanism for reporting such errors has been reimplemented
1741  to queue compile-time errors and report them at the end of the
1742  compilation as true errors rather than as warnings.  This fixes
1743  cases where error messages leaked through in the form of warnings
1744  when code was compiled at run time using C<eval STRING>, and
1745  also allows such errors to be reliably trapped using C<eval "...">.
1746  
1747  =head2 Implicitly closed filehandles are safer
1748  
1749  Sometimes implicitly closed filehandles (as when they are localized,
1750  and Perl automatically closes them on exiting the scope) could
1751  inadvertently set $? or $!.  This has been corrected.
1752  
1753  
1754  =head2 Behavior of list slices is more consistent
1755  
1756  When taking a slice of a literal list (as opposed to a slice of
1757  an array or hash), Perl used to return an empty list if the
1758  result happened to be composed of all undef values.
1759  
1760  The new behavior is to produce an empty list if (and only if)
1761  the original list was empty.  Consider the following example:
1762  
1763      @a = (1,undef,undef,2)[2,1,2];
1764  
1765  The old behavior would have resulted in @a having no elements.
1766  The new behavior ensures it has three undefined elements.
1767  
1768  Note in particular that the behavior of slices of the following
1769  cases remains unchanged:
1770  
1771      @a = ()[1,2];
1772      @a = (getpwent)[7,0];
1773      @a = (anything_returning_empty_list())[2,1,2];
1774      @a = @b[2,1,2];
1775      @a = @c{'a','b','c'};
1776  
1777  See L<perldata>.
1778  
1779  =head2 C<(\$)> prototype and C<$foo{a}>
1780  
1781  A scalar reference prototype now correctly allows a hash or
1782  array element in that slot.
1783  
1784  =head2 C<goto &sub> and AUTOLOAD
1785  
1786  The C<goto &sub> construct works correctly when C<&sub> happens
1787  to be autoloaded.
1788  
1789  =head2 C<-bareword> allowed under C<use integer>
1790  
1791  The autoquoting of barewords preceded by C<-> did not work
1792  in prior versions when the C<integer> pragma was enabled.
1793  This has been fixed.
1794  
1795  =head2 Failures in DESTROY()
1796  
1797  When code in a destructor threw an exception, it went unnoticed
1798  in earlier versions of Perl, unless someone happened to be
1799  looking in $@ just after the point the destructor happened to
1800  run.  Such failures are now visible as warnings when warnings are
1801  enabled.
1802  
1803  =head2 Locale bugs fixed
1804  
1805  printf() and sprintf() previously reset the numeric locale
1806  back to the default "C" locale.  This has been fixed.
1807  
1808  Numbers formatted according to the local numeric locale
1809  (such as using a decimal comma instead of a decimal dot) caused
1810  "isn't numeric" warnings, even while the operations accessing
1811  those numbers produced correct results.  These warnings have been
1812  discontinued.
1813  
1814  =head2 Memory leaks
1815  
1816  The C<eval 'return sub {...}'> construct could sometimes leak
1817  memory.  This has been fixed.
1818  
1819  Operations that aren't filehandle constructors used to leak memory
1820  when used on invalid filehandles.  This has been fixed.
1821  
1822  Constructs that modified C<@_> could fail to deallocate values
1823  in C<@_> and thus leak memory.  This has been corrected.
1824  
1825  =head2 Spurious subroutine stubs after failed subroutine calls
1826  
1827  Perl could sometimes create empty subroutine stubs when a
1828  subroutine was not found in the package.  Such cases stopped
1829  later method lookups from progressing into base packages.
1830  This has been corrected.
1831  
1832  =head2 Taint failures under C<-U>
1833  
1834  When running in unsafe mode, taint violations could sometimes
1835  cause silent failures.  This has been fixed.
1836  
1837  =head2 END blocks and the C<-c> switch
1838  
1839  Prior versions used to run BEGIN B<and> END blocks when Perl was
1840  run in compile-only mode.  Since this is typically not the expected
1841  behavior, END blocks are not executed anymore when the C<-c> switch
1842  is used, or if compilation fails.
1843  
1844  See L</"Support for CHECK blocks"> for how to run things when the compile 
1845  phase ends.
1846  
1847  =head2 Potential to leak DATA filehandles
1848  
1849  Using the C<__DATA__> token creates an implicit filehandle to
1850  the file that contains the token.  It is the program's
1851  responsibility to close it when it is done reading from it.
1852  
1853  This caveat is now better explained in the documentation.
1854  See L<perldata>.
1855  
1856  =head1 New or Changed Diagnostics
1857  
1858  =over 4
1859  
1860  =item "%s" variable %s masks earlier declaration in same %s
1861  
1862  (W misc) A "my" or "our" variable has been redeclared in the current scope or statement,
1863  effectively eliminating all access to the previous instance.  This is almost
1864  always a typographical error.  Note that the earlier variable will still exist
1865  until the end of the scope or until all closure referents to it are
1866  destroyed.
1867  
1868  =item "my sub" not yet implemented
1869  
1870  (F) Lexically scoped subroutines are not yet implemented.  Don't try that
1871  yet.
1872  
1873  =item "our" variable %s redeclared
1874  
1875  (W misc) You seem to have already declared the same global once before in the
1876  current lexical scope.
1877  
1878  =item '!' allowed only after types %s
1879  
1880  (F) The '!' is allowed in pack() and unpack() only after certain types.
1881  See L<perlfunc/pack>.
1882  
1883  =item / cannot take a count
1884  
1885  (F) You had an unpack template indicating a counted-length string,
1886  but you have also specified an explicit size for the string.
1887  See L<perlfunc/pack>.
1888  
1889  =item / must be followed by a, A or Z
1890  
1891  (F) You had an unpack template indicating a counted-length string,
1892  which must be followed by one of the letters a, A or Z
1893  to indicate what sort of string is to be unpacked.
1894  See L<perlfunc/pack>.
1895  
1896  =item / must be followed by a*, A* or Z*
1897  
1898  (F) You had a pack template indicating a counted-length string,
1899  Currently the only things that can have their length counted are a*, A* or Z*.
1900  See L<perlfunc/pack>.
1901  
1902  =item / must follow a numeric type
1903  
1904  (F) You had an unpack template that contained a '#',
1905  but this did not follow some numeric unpack specification.
1906  See L<perlfunc/pack>.
1907  
1908  =item /%s/: Unrecognized escape \\%c passed through
1909  
1910  (W regexp) You used a backslash-character combination which is not recognized
1911  by Perl.  This combination appears in an interpolated variable or a
1912  C<'>-delimited regular expression.  The character was understood literally.
1913  
1914  =item /%s/: Unrecognized escape \\%c in character class passed through
1915  
1916  (W regexp) You used a backslash-character combination which is not recognized
1917  by Perl inside character classes.  The character was understood literally.
1918  
1919  =item /%s/ should probably be written as "%s"
1920  
1921  (W syntax) You have used a pattern where Perl expected to find a string,
1922  as in the first argument to C<join>.  Perl will treat the true
1923  or false result of matching the pattern against $_ as the string,
1924  which is probably not what you had in mind.
1925  
1926  =item %s() called too early to check prototype
1927  
1928  (W prototype) You've called a function that has a prototype before the parser saw a
1929  definition or declaration for it, and Perl could not check that the call
1930  conforms to the prototype.  You need to either add an early prototype
1931  declaration for the subroutine in question, or move the subroutine
1932  definition ahead of the call to get proper prototype checking.  Alternatively,
1933  if you are certain that you're calling the function correctly, you may put
1934  an ampersand before the name to avoid the warning.  See L<perlsub>.
1935  
1936  =item %s argument is not a HASH or ARRAY element
1937  
1938  (F) The argument to exists() must be a hash or array element, such as:
1939  
1940      $foo{$bar}
1941      $ref->{"susie"}[12]
1942  
1943  =item %s argument is not a HASH or ARRAY element or slice
1944  
1945  (F) The argument to delete() must be either a hash or array element, such as:
1946  
1947      $foo{$bar}
1948      $ref->{"susie"}[12]
1949  
1950  or a hash or array slice, such as:
1951  
1952      @foo[$bar, $baz, $xyzzy]
1953      @{$ref->[12]}{"susie", "queue"}
1954  
1955  =item %s argument is not a subroutine name
1956  
1957  (F) The argument to exists() for C<exists &sub> must be a subroutine
1958  name, and not a subroutine call.  C<exists &sub()> will generate this error.
1959  
1960  =item %s package attribute may clash with future reserved word: %s
1961  
1962  (W reserved) A lowercase attribute name was used that had a package-specific handler.
1963  That name might have a meaning to Perl itself some day, even though it
1964  doesn't yet.  Perhaps you should use a mixed-case attribute name, instead.
1965  See L<attributes>.
1966  
1967  =item (in cleanup) %s
1968  
1969  (W misc) This prefix usually indicates that a DESTROY() method raised
1970  the indicated exception.  Since destructors are usually called by
1971  the system at arbitrary points during execution, and often a vast
1972  number of times, the warning is issued only once for any number
1973  of failures that would otherwise result in the same message being
1974  repeated.
1975  
1976  Failure of user callbacks dispatched using the C<G_KEEPERR> flag
1977  could also result in this warning.  See L<perlcall/G_KEEPERR>.
1978  
1979  =item <> should be quotes
1980  
1981  (F) You wrote C<< require <file> >> when you should have written
1982  C<require 'file'>.
1983  
1984  =item Attempt to join self
1985  
1986  (F) You tried to join a thread from within itself, which is an
1987  impossible task.  You may be joining the wrong thread, or you may
1988  need to move the join() to some other thread.
1989  
1990  =item Bad evalled substitution pattern
1991  
1992  (F) You've used the /e switch to evaluate the replacement for a
1993  substitution, but perl found a syntax error in the code to evaluate,
1994  most likely an unexpected right brace '}'.
1995  
1996  =item Bad realloc() ignored
1997  
1998  (S) An internal routine called realloc() on something that had never been
1999  malloc()ed in the first place. Mandatory, but can be disabled by
2000  setting environment variable C<PERL_BADFREE> to 1.
2001  
2002  =item Bareword found in conditional
2003  
2004  (W bareword) The compiler found a bareword where it expected a conditional,
2005  which often indicates that an || or && was parsed as part of the
2006  last argument of the previous construct, for example:
2007  
2008      open FOO || die;
2009  
2010  It may also indicate a misspelled constant that has been interpreted
2011  as a bareword:
2012  
2013      use constant TYPO => 1;
2014      if (TYOP) { print "foo" }
2015  
2016  The C<strict> pragma is useful in avoiding such errors.
2017  
2018  =item Binary number > 0b11111111111111111111111111111111 non-portable
2019  
2020  (W portable) The binary number you specified is larger than 2**32-1
2021  (4294967295) and therefore non-portable between systems.  See
2022  L<perlport> for more on portability concerns.
2023  
2024  =item Bit vector size > 32 non-portable
2025  
2026  (W portable) Using bit vector sizes larger than 32 is non-portable.
2027  
2028  =item Buffer overflow in prime_env_iter: %s
2029  
2030  (W internal) A warning peculiar to VMS.  While Perl was preparing to iterate over
2031  %ENV, it encountered a logical name or symbol definition which was too long,
2032  so it was truncated to the string shown.
2033  
2034  =item Can't check filesystem of script "%s"
2035  
2036  (P) For some reason you can't check the filesystem of the script for nosuid.
2037  
2038  =item Can't declare class for non-scalar %s in "%s"
2039  
2040  (S) Currently, only scalar variables can declared with a specific class
2041  qualifier in a "my" or "our" declaration.  The semantics may be extended
2042  for other types of variables in future.
2043  
2044  =item Can't declare %s in "%s"
2045  
2046  (F) Only scalar, array, and hash variables may be declared as "my" or
2047  "our" variables.  They must have ordinary identifiers as names.
2048  
2049  =item Can't ignore signal CHLD, forcing to default
2050  
2051  (W signal) Perl has detected that it is being run with the SIGCHLD signal
2052  (sometimes known as SIGCLD) disabled.  Since disabling this signal
2053  will interfere with proper determination of exit status of child
2054  processes, Perl has reset the signal to its default value.
2055  This situation typically indicates that the parent program under
2056  which Perl may be running (e.g., cron) is being very careless.
2057  
2058  =item Can't modify non-lvalue subroutine call
2059  
2060  (F) Subroutines meant to be used in lvalue context should be declared as
2061  such, see L<perlsub/"Lvalue subroutines">.
2062  
2063  =item Can't read CRTL environ
2064  
2065  (S) A warning peculiar to VMS.  Perl tried to read an element of %ENV
2066  from the CRTL's internal environment array and discovered the array was
2067  missing.  You need to figure out where your CRTL misplaced its environ
2068  or define F<PERL_ENV_TABLES> (see L<perlvms>) so that environ is not searched.
2069  
2070  =item Can't remove %s: %s, skipping file 
2071  
2072  (S) You requested an inplace edit without creating a backup file.  Perl
2073  was unable to remove the original file to replace it with the modified
2074  file.  The file was left unmodified.
2075  
2076  =item Can't return %s from lvalue subroutine
2077  
2078  (F) Perl detected an attempt to return illegal lvalues (such
2079  as temporary or readonly values) from a subroutine used as an lvalue.
2080  This is not allowed.
2081  
2082  =item Can't weaken a nonreference
2083  
2084  (F) You attempted to weaken something that was not a reference.  Only
2085  references can be weakened.
2086  
2087  =item Character class [:%s:] unknown
2088  
2089  (F) The class in the character class [: :] syntax is unknown.
2090  See L<perlre>.
2091  
2092  =item Character class syntax [%s] belongs inside character classes
2093  
2094  (W unsafe) The character class constructs [: :], [= =], and [. .]  go
2095  I<inside> character classes, the [] are part of the construct,
2096  for example: /[012[:alpha:]345]/.  Note that [= =] and [. .]
2097  are not currently implemented; they are simply placeholders for
2098  future extensions.
2099  
2100  =item Constant is not %s reference
2101  
2102  (F) A constant value (perhaps declared using the C<use constant> pragma)
2103  is being dereferenced, but it amounts to the wrong type of reference.  The
2104  message indicates the type of reference that was expected. This usually
2105  indicates a syntax error in dereferencing the constant value.
2106  See L<perlsub/"Constant Functions"> and L<constant>.
2107  
2108  =item constant(%s): %s
2109  
2110  (F) The parser found inconsistencies either while attempting to define an
2111  overloaded constant, or when trying to find the character name specified
2112  in the C<\N{...}> escape.  Perhaps you forgot to load the corresponding
2113  C<overload> or C<charnames> pragma?  See L<charnames> and L<overload>.
2114  
2115  =item CORE::%s is not a keyword
2116  
2117  (F) The CORE:: namespace is reserved for Perl keywords.
2118  
2119  =item defined(@array) is deprecated
2120  
2121  (D) defined() is not usually useful on arrays because it checks for an
2122  undefined I<scalar> value.  If you want to see if the array is empty,
2123  just use C<if (@array) { # not empty }> for example.  
2124  
2125  =item defined(%hash) is deprecated
2126  
2127  (D) defined() is not usually useful on hashes because it checks for an
2128  undefined I<scalar> value.  If you want to see if the hash is empty,
2129  just use C<if (%hash) { # not empty }> for example.  
2130  
2131  =item Did not produce a valid header
2132  
2133  See Server error.
2134  
2135  =item (Did you mean "local" instead of "our"?)
2136  
2137  (W misc) Remember that "our" does not localize the declared global variable.
2138  You have declared it again in the same lexical scope, which seems superfluous.
2139  
2140  =item Document contains no data
2141  
2142  See Server error.
2143  
2144  =item entering effective %s failed
2145  
2146  (F) While under the C<use filetest> pragma, switching the real and
2147  effective uids or gids failed.
2148  
2149  =item false [] range "%s" in regexp
2150  
2151  (W regexp) A character class range must start and end at a literal character, not
2152  another character class like C<\d> or C<[:alpha:]>.  The "-" in your false
2153  range is interpreted as a literal "-".  Consider quoting the "-",  "\-".
2154  See L<perlre>.
2155  
2156  =item Filehandle %s opened only for output
2157  
2158  (W io) You tried to read from a filehandle opened only for writing.  If you
2159  intended it to be a read/write filehandle, you needed to open it with
2160  "+<" or "+>" or "+>>" instead of with "<" or nothing.  If
2161  you intended only to read from the file, use "<".  See
2162  L<perlfunc/open>.
2163  
2164  =item flock() on closed filehandle %s
2165  
2166  (W closed) The filehandle you're attempting to flock() got itself closed some
2167  time before now.  Check your logic flow.  flock() operates on filehandles.
2168  Are you attempting to call flock() on a dirhandle by the same name?
2169  
2170  =item Global symbol "%s" requires explicit package name
2171  
2172  (F) You've said "use strict vars", which indicates that all variables
2173  must either be lexically scoped (using "my"), declared beforehand using
2174  "our", or explicitly qualified to say which package the global variable
2175  is in (using "::").
2176  
2177  =item Hexadecimal number > 0xffffffff non-portable
2178  
2179  (W portable) The hexadecimal number you specified is larger than 2**32-1
2180  (4294967295) and therefore non-portable between systems.  See
2181  L<perlport> for more on portability concerns.
2182  
2183  =item Ill-formed CRTL environ value "%s"
2184  
2185  (W internal) A warning peculiar to VMS.  Perl tried to read the CRTL's internal
2186  environ array, and encountered an element without the C<=> delimiter
2187  used to separate keys from values.  The element is ignored.
2188  
2189  =item Ill-formed message in prime_env_iter: |%s|
2190  
2191  (W internal) A warning peculiar to VMS.  Perl tried to read a logical name
2192  or CLI symbol definition when preparing to iterate over %ENV, and
2193  didn't see the expected delimiter between key and value, so the
2194  line was ignored.
2195  
2196  =item Illegal binary digit %s
2197  
2198  (F) You used a digit other than 0 or 1 in a binary number.
2199  
2200  =item Illegal binary digit %s ignored
2201  
2202  (W digit) You may have tried to use a digit other than 0 or 1 in a binary number.
2203  Interpretation of the binary number stopped before the offending digit.
2204  
2205  =item Illegal number of bits in vec
2206  
2207  (F) The number of bits in vec() (the third argument) must be a power of
2208  two from 1 to 32 (or 64, if your platform supports that).
2209  
2210  =item Integer overflow in %s number
2211  
2212  (W overflow) The hexadecimal, octal or binary number you have specified either
2213  as a literal or as an argument to hex() or oct() is too big for your
2214  architecture, and has been converted to a floating point number.  On a
2215  32-bit architecture the largest hexadecimal, octal or binary number
2216  representable without overflow is 0xFFFFFFFF, 037777777777, or
2217  0b11111111111111111111111111111111 respectively.  Note that Perl
2218  transparently promotes all numbers to a floating point representation
2219  internally--subject to loss of precision errors in subsequent
2220  operations.
2221  
2222  =item Invalid %s attribute: %s
2223  
2224  The indicated attribute for a subroutine or variable was not recognized
2225  by Perl or by a user-supplied handler.  See L<attributes>.
2226  
2227  =item Invalid %s attributes: %s
2228  
2229  The indicated attributes for a subroutine or variable were not recognized
2230  by Perl or by a user-supplied handler.  See L<attributes>.
2231  
2232  =item invalid [] range "%s" in regexp
2233  
2234  The offending range is now explicitly displayed.
2235  
2236  =item Invalid separator character %s in attribute list
2237  
2238  (F) Something other than a colon or whitespace was seen between the
2239  elements of an attribute list.  If the previous attribute
2240  had a parenthesised parameter list, perhaps that list was terminated
2241  too soon.  See L<attributes>.
2242  
2243  =item Invalid separator character %s in subroutine attribute list
2244  
2245  (F) Something other than a colon or whitespace was seen between the
2246  elements of a subroutine attribute list.  If the previous attribute
2247  had a parenthesised parameter list, perhaps that list was terminated
2248  too soon.
2249  
2250  =item leaving effective %s failed
2251  
2252  (F) While under the C<use filetest> pragma, switching the real and
2253  effective uids or gids failed.
2254  
2255  =item Lvalue subs returning %s not implemented yet
2256  
2257  (F) Due to limitations in the current implementation, array and hash
2258  values cannot be returned in subroutines used in lvalue context.
2259  See L<perlsub/"Lvalue subroutines">.
2260  
2261  =item Method %s not permitted
2262  
2263  See Server error.
2264  
2265  =item Missing %sbrace%s on \N{}
2266  
2267  (F) Wrong syntax of character name literal C<\N{charname}> within
2268  double-quotish context.
2269  
2270  =item Missing command in piped open
2271  
2272  (W pipe) You used the C<open(FH, "| command")> or C<open(FH, "command |")>
2273  construction, but the command was missing or blank.
2274  
2275  =item Missing name in "my sub"
2276  
2277  (F) The reserved syntax for lexically scoped subroutines requires that they
2278  have a name with which they can be found.
2279  
2280  =item No %s specified for -%c
2281  
2282  (F) The indicated command line switch needs a mandatory argument, but
2283  you haven't specified one.
2284  
2285  =item No package name allowed for variable %s in "our"
2286  
2287  (F) Fully qualified variable names are not allowed in "our" declarations,
2288  because that doesn't make much sense under existing semantics.  Such
2289  syntax is reserved for future extensions.
2290  
2291  =item No space allowed after -%c
2292  
2293  (F) The argument to the indicated command line switch must follow immediately
2294  after the switch, without intervening spaces.
2295  
2296  =item no UTC offset information; assuming local time is UTC
2297  
2298  (S) A warning peculiar to VMS.  Perl was unable to find the local
2299  timezone offset, so it's assuming that local system time is equivalent
2300  to UTC.  If it's not, define the logical name F<SYS$TIMEZONE_DIFFERENTIAL>
2301  to translate to the number of seconds which need to be added to UTC to
2302  get local time.
2303  
2304  =item Octal number > 037777777777 non-portable
2305  
2306  (W portable) The octal number you specified is larger than 2**32-1 (4294967295)
2307  and therefore non-portable between systems.  See L<perlport> for more
2308  on portability concerns.
2309  
2310  See also L<perlport> for writing portable code.
2311  
2312  =item panic: del_backref
2313  
2314  (P) Failed an internal consistency check while trying to reset a weak
2315  reference.
2316  
2317  =item panic: kid popen errno read
2318  
2319  (F) forked child returned an incomprehensible message about its errno.
2320  
2321  =item panic: magic_killbackrefs
2322  
2323  (P) Failed an internal consistency check while trying to reset all weak
2324  references to an object.
2325  
2326  =item Parentheses missing around "%s" list
2327  
2328  (W parenthesis) You said something like
2329  
2330      my $foo, $bar = @_;
2331  
2332  when you meant
2333  
2334      my ($foo, $bar) = @_;
2335  
2336  Remember that "my", "our", and "local" bind tighter than comma.
2337  
2338  =item Possible unintended interpolation of %s in string
2339  
2340  (W ambiguous) It used to be that Perl would try to guess whether you
2341  wanted an array interpolated or a literal @.  It no longer does this;
2342  arrays are now I<always> interpolated into strings.  This means that 
2343  if you try something like:
2344  
2345          print "fred@example.com";
2346  
2347  and the array C<@example> doesn't exist, Perl is going to print
2348  C<fred.com>, which is probably not what you wanted.  To get a literal
2349  C<@> sign in a string, put a backslash before it, just as you would
2350  to get a literal C<$> sign.
2351  
2352  =item Possible Y2K bug: %s
2353  
2354  (W y2k) You are concatenating the number 19 with another number, which
2355  could be a potential Year 2000 problem.
2356  
2357  =item pragma "attrs" is deprecated, use "sub NAME : ATTRS" instead
2358  
2359  (W deprecated) You have written something like this:
2360  
2361      sub doit
2362      {
2363          use attrs qw(locked);
2364      }
2365  
2366  You should use the new declaration syntax instead.
2367  
2368      sub doit : locked
2369      {
2370          ...
2371  
2372  The C<use attrs> pragma is now obsolete, and is only provided for
2373  backward-compatibility. See L<perlsub/"Subroutine Attributes">.
2374  
2375  
2376  =item Premature end of script headers
2377  
2378  See Server error.
2379  
2380  =item Repeat count in pack overflows
2381  
2382  (F) You can't specify a repeat count so large that it overflows
2383  your signed integers.  See L<perlfunc/pack>.
2384  
2385  =item Repeat count in unpack overflows
2386  
2387  (F) You can't specify a repeat count so large that it overflows
2388  your signed integers.  See L<perlfunc/unpack>.
2389  
2390  =item realloc() of freed memory ignored
2391  
2392  (S) An internal routine called realloc() on something that had already
2393  been freed.
2394  
2395  =item Reference is already weak
2396  
2397  (W misc) You have attempted to weaken a reference that is already weak.
2398  Doing so has no effect.
2399  
2400  =item setpgrp can't take arguments
2401  
2402  (F) Your system has the setpgrp() from BSD 4.2, which takes no arguments,
2403  unlike POSIX setpgid(), which takes a process ID and process group ID.
2404  
2405  =item Strange *+?{} on zero-length expression
2406  
2407  (W regexp) You applied a regular expression quantifier in a place where it
2408  makes no sense, such as on a zero-width assertion.
2409  Try putting the quantifier inside the assertion instead.  For example,
2410  the way to match "abc" provided that it is followed by three
2411  repetitions of "xyz" is C</abc(?=(?:xyz){3})/>, not C</abc(?=xyz){3}/>.
2412  
2413  =item switching effective %s is not implemented
2414  
2415  (F) While under the C<use filetest> pragma, we cannot switch the
2416  real and effective uids or gids.
2417  
2418  =item This Perl can't reset CRTL environ elements (%s)
2419  
2420  =item This Perl can't set CRTL environ elements (%s=%s)
2421  
2422  (W internal) Warnings peculiar to VMS.  You tried to change or delete an element
2423  of the CRTL's internal environ array, but your copy of Perl wasn't
2424  built with a CRTL that contained the setenv() function.  You'll need to
2425  rebuild Perl with a CRTL that does, or redefine F<PERL_ENV_TABLES> (see
2426  L<perlvms>) so that the environ array isn't the target of the change to
2427  %ENV which produced the warning.
2428  
2429  =item Too late to run %s block
2430  
2431  (W void) A CHECK or INIT block is being defined during run time proper,
2432  when the opportunity to run them has already passed.  Perhaps you are
2433  loading a file with C<require> or C<do> when you should be using
2434  C<use> instead.  Or perhaps you should put the C<require> or C<do>
2435  inside a BEGIN block.
2436  
2437  =item Unknown open() mode '%s'
2438  
2439  (F) The second argument of 3-argument open() is not among the list
2440  of valid modes: C<< < >>, C<< > >>, C<<< >> >>>, C<< +< >>,
2441  C<< +> >>, C<<< +>> >>>, C<-|>, C<|->.
2442  
2443  =item Unknown process %x sent message to prime_env_iter: %s
2444  
2445  (P) An error peculiar to VMS.  Perl was reading values for %ENV before
2446  iterating over it, and someone else stuck a message in the stream of
2447  data Perl expected.  Someone's very confused, or perhaps trying to
2448  subvert Perl's population of %ENV for nefarious purposes.
2449  
2450  =item Unrecognized escape \\%c passed through
2451  
2452  (W misc) You used a backslash-character combination which is not recognized
2453  by Perl.  The character was understood literally.
2454  
2455  =item Unterminated attribute parameter in attribute list
2456  
2457  (F) The lexer saw an opening (left) parenthesis character while parsing an
2458  attribute list, but the matching closing (right) parenthesis
2459  character was not found.  You may need to add (or remove) a backslash
2460  character to get your parentheses to balance.  See L<attributes>.
2461  
2462  =item Unterminated attribute list
2463  
2464  (F) The lexer found something other than a simple identifier at the start
2465  of an attribute, and it wasn't a semicolon or the start of a
2466  block.  Perhaps you terminated the parameter list of the previous attribute
2467  too soon.  See L<attributes>.
2468  
2469  =item Unterminated attribute parameter in subroutine attribute list
2470  
2471  (F) The lexer saw an opening (left) parenthesis character while parsing a
2472  subroutine attribute list, but the matching closing (right) parenthesis
2473  character was not found.  You may need to add (or remove) a backslash
2474  character to get your parentheses to balance.
2475  
2476  =item Unterminated subroutine attribute list
2477  
2478  (F) The lexer found something other than a simple identifier at the start
2479  of a subroutine attribute, and it wasn't a semicolon or the start of a
2480  block.  Perhaps you terminated the parameter list of the previous attribute
2481  too soon.
2482  
2483  =item Value of CLI symbol "%s" too long
2484  
2485  (W misc) A warning peculiar to VMS.  Perl tried to read the value of an %ENV
2486  element from a CLI symbol table, and found a resultant string longer
2487  than 1024 characters.  The return value has been truncated to 1024
2488  characters.
2489  
2490  =item Version number must be a constant number
2491  
2492  (P) The attempt to translate a C<use Module n.n LIST> statement into
2493  its equivalent C<BEGIN> block found an internal inconsistency with
2494  the version number.
2495  
2496  =back
2497  
2498  =head1 New tests
2499  
2500  =over 4
2501  
2502  =item    lib/attrs
2503  
2504  Compatibility tests for C<sub : attrs> vs the older C<use attrs>.
2505  
2506  =item    lib/env
2507  
2508  Tests for new environment scalar capability (e.g., C<use Env qw($BAR);>).
2509  
2510  =item    lib/env-array
2511  
2512  Tests for new environment array capability (e.g., C<use Env qw(@PATH);>).
2513  
2514  =item    lib/io_const
2515  
2516  IO constants (SEEK_*, _IO*).
2517  
2518  =item    lib/io_dir
2519  
2520  Directory-related IO methods (new, read, close, rewind, tied delete).
2521  
2522  =item    lib/io_multihomed
2523  
2524  INET sockets with multi-homed hosts.
2525  
2526  =item    lib/io_poll
2527  
2528  IO poll().
2529  
2530  =item    lib/io_unix
2531  
2532  UNIX sockets.
2533  
2534  =item    op/attrs
2535  
2536  Regression tests for C<my ($x,@y,%z) : attrs> and <sub : attrs>.
2537  
2538  =item    op/filetest
2539  
2540  File test operators.
2541  
2542  =item    op/lex_assign
2543  
2544  Verify operations that access pad objects (lexicals and temporaries).
2545  
2546  =item    op/exists_sub
2547  
2548  Verify C<exists &sub> operations.
2549  
2550  =back
2551  
2552  =head1 Incompatible Changes
2553  
2554  =head2 Perl Source Incompatibilities
2555  
2556  Beware that any new warnings that have been added or old ones
2557  that have been enhanced are B<not> considered incompatible changes.
2558  
2559  Since all new warnings must be explicitly requested via the C<-w>
2560  switch or the C<warnings> pragma, it is ultimately the programmer's
2561  responsibility to ensure that warnings are enabled judiciously.
2562  
2563  =over 4
2564  
2565  =item CHECK is a new keyword
2566  
2567  All subroutine definitions named CHECK are now special.  See
2568  C</"Support for CHECK blocks"> for more information.
2569  
2570  =item Treatment of list slices of undef has changed
2571  
2572  There is a potential incompatibility in the behavior of list slices
2573  that are comprised entirely of undefined values.
2574  See L</"Behavior of list slices is more consistent">.
2575  
2576  =item Format of $English::PERL_VERSION is different
2577  
2578  The English module now sets $PERL_VERSION to $^V (a string value) rather
2579  than C<$]> (a numeric value).  This is a potential incompatibility.
2580  Send us a report via perlbug if you are affected by this.
2581  
2582  See L</"Improved Perl version numbering system"> for the reasons for
2583  this change.
2584  
2585  =item Literals of the form C<1.2.3> parse differently
2586  
2587  Previously, numeric literals with more than one dot in them were
2588  interpreted as a floating point number concatenated with one or more
2589  numbers.  Such "numbers" are now parsed as strings composed of the
2590  specified ordinals.
2591  
2592  For example, C<print 97.98.99> used to output C<97.9899> in earlier
2593  versions, but now prints C<abc>.
2594  
2595  See L</"Support for strings represented as a vector of ordinals">.
2596  
2597  =item Possibly changed pseudo-random number generator
2598  
2599  Perl programs that depend on reproducing a specific set of pseudo-random
2600  numbers may now produce different output due to improvements made to the
2601  rand() builtin.  You can use C<sh Configure -Drandfunc=rand> to obtain
2602  the old behavior.
2603  
2604  See L</"Better pseudo-random number generator">.
2605  
2606  =item Hashing function for hash keys has changed
2607  
2608  Even though Perl hashes are not order preserving, the apparently
2609  random order encountered when iterating on the contents of a hash
2610  is actually determined by the hashing algorithm used.  Improvements
2611  in the algorithm may yield a random order that is B<different> from
2612  that of previous versions, especially when iterating on hashes.
2613  
2614  See L</"Better worst-case behavior of hashes"> for additional
2615  information.
2616  
2617  =item C<undef> fails on read only values
2618  
2619  Using the C<undef> operator on a readonly value (such as $1) has
2620  the same effect as assigning C<undef> to the readonly value--it
2621  throws an exception.
2622  
2623  =item Close-on-exec bit may be set on pipe and socket handles
2624  
2625  Pipe and socket handles are also now subject to the close-on-exec
2626  behavior determined by the special variable $^F.
2627  
2628  See L</"More consistent close-on-exec behavior">.
2629  
2630  =item Writing C<"$$1"> to mean C<"${$}1"> is unsupported
2631  
2632  Perl 5.004 deprecated the interpretation of C<$$1> and
2633  similar within interpolated strings to mean C<$$ . "1">,
2634  but still allowed it.
2635  
2636  In Perl 5.6.0 and later, C<"$$1"> always means C<"${$1}">.
2637  
2638  =item delete(), each(), values() and C<\(%h)>
2639  
2640  operate on aliases to values, not copies
2641  
2642  delete(), each(), values() and hashes (e.g. C<\(%h)>)
2643  in a list context return the actual
2644  values in the hash, instead of copies (as they used to in earlier
2645  versions).  Typical idioms for using these constructs copy the
2646  returned values, but this can make a significant difference when
2647  creating references to the returned values.  Keys in the hash are still
2648  returned as copies when iterating on a hash.
2649  
2650  See also L</"delete(), each(), values() and hash iteration are faster">.
2651  
2652  =item vec(EXPR,OFFSET,BITS) enforces powers-of-two BITS
2653  
2654  vec() generates a run-time error if the BITS argument is not
2655  a valid power-of-two integer.
2656  
2657  =item Text of some diagnostic output has changed
2658  
2659  Most references to internal Perl operations in diagnostics
2660  have been changed to be more descriptive.  This may be an
2661  issue for programs that may incorrectly rely on the exact
2662  text of diagnostics for proper functioning.
2663  
2664  =item C<%@> has been removed
2665  
2666  The undocumented special variable C<%@> that used to accumulate
2667  "background" errors (such as those that happen in DESTROY())
2668  has been removed, because it could potentially result in memory
2669  leaks.
2670  
2671  =item Parenthesized not() behaves like a list operator
2672  
2673  The C<not> operator now falls under the "if it looks like a function,
2674  it behaves like a function" rule.
2675  
2676  As a result, the parenthesized form can be used with C<grep> and C<map>.
2677  The following construct used to be a syntax error before, but it works
2678  as expected now:
2679  
2680      grep not($_), @things;
2681  
2682  On the other hand, using C<not> with a literal list slice may not
2683  work.  The following previously allowed construct:
2684  
2685      print not (1,2,3)[0];
2686  
2687  needs to be written with additional parentheses now:
2688  
2689      print not((1,2,3)[0]);
2690  
2691  The behavior remains unaffected when C<not> is not followed by parentheses.
2692  
2693  =item Semantics of bareword prototype C<(*)> have changed
2694  
2695  The semantics of the bareword prototype C<*> have changed.  Perl 5.005
2696  always coerced simple scalar arguments to a typeglob, which wasn't useful
2697  in situations where the subroutine must distinguish between a simple
2698  scalar and a typeglob.  The new behavior is to not coerce bareword
2699  arguments to a typeglob.  The value will always be visible as either
2700  a simple scalar or as a reference to a typeglob.
2701  
2702  See L</"More functional bareword prototype (*)">.
2703  
2704  =item Semantics of bit operators may have changed on 64-bit platforms
2705  
2706  If your platform is either natively 64-bit or if Perl has been
2707  configured to used 64-bit integers, i.e., $Config{ivsize} is 8, 
2708  there may be a potential incompatibility in the behavior of bitwise
2709  numeric operators (& | ^ ~ << >>).  These operators used to strictly
2710  operate on the lower 32 bits of integers in previous versions, but now
2711  operate over the entire native integral width.  In particular, note
2712  that unary C<~> will produce different results on platforms that have
2713  different $Config{ivsize}.  For portability, be sure to mask off
2714  the excess bits in the result of unary C<~>, e.g., C<~$x & 0xffffffff>.
2715  
2716  See L</"Bit operators support full native integer width">.
2717  
2718  =item More builtins taint their results
2719  
2720  As described in L</"Improved security features">, there may be more
2721  sources of taint in a Perl program.
2722  
2723  To avoid these new tainting behaviors, you can build Perl with the
2724  Configure option C<-Accflags=-DINCOMPLETE_TAINTS>.  Beware that the
2725  ensuing perl binary may be insecure.
2726  
2727  =back
2728  
2729  =head2 C Source Incompatibilities
2730  
2731  =over 4
2732  
2733  =item C<PERL_POLLUTE>
2734  
2735  Release 5.005 grandfathered old global symbol names by providing preprocessor
2736  macros for extension source compatibility.  As of release 5.6.0, these
2737  preprocessor definitions are not available by default.  You need to explicitly
2738  compile perl with C<-DPERL_POLLUTE> to get these definitions.  For
2739  extensions still using the old symbols, this option can be
2740  specified via MakeMaker:
2741  
2742      perl Makefile.PL POLLUTE=1
2743  
2744  =item C<PERL_IMPLICIT_CONTEXT>
2745  
2746  This new build option provides a set of macros for all API functions
2747  such that an implicit interpreter/thread context argument is passed to
2748  every API function.  As a result of this, something like C<sv_setsv(foo,bar)>
2749  amounts to a macro invocation that actually translates to something like
2750  C<Perl_sv_setsv(my_perl,foo,bar)>.  While this is generally expected
2751  to not have any significant source compatibility issues, the difference
2752  between a macro and a real function call will need to be considered.
2753  
2754  This means that there B<is> a source compatibility issue as a result of
2755  this if your extensions attempt to use pointers to any of the Perl API
2756  functions.
2757  
2758  Note that the above issue is not relevant to the default build of
2759  Perl, whose interfaces continue to match those of prior versions
2760  (but subject to the other options described here).
2761  
2762  See L<perlguts/"The Perl API"> for detailed information on the
2763  ramifications of building Perl with this option.
2764  
2765      NOTE: PERL_IMPLICIT_CONTEXT is automatically enabled whenever Perl is built
2766      with one of -Dusethreads, -Dusemultiplicity, or both.  It is not
2767      intended to be enabled by users at this time.
2768  
2769  =item C<PERL_POLLUTE_MALLOC>
2770  
2771  Enabling Perl's malloc in release 5.005 and earlier caused the namespace of
2772  the system's malloc family of functions to be usurped by the Perl versions,
2773  since by default they used the same names.  Besides causing problems on
2774  platforms that do not allow these functions to be cleanly replaced, this
2775  also meant that the system versions could not be called in programs that
2776  used Perl's malloc.  Previous versions of Perl have allowed this behaviour
2777  to be suppressed with the HIDEMYMALLOC and EMBEDMYMALLOC preprocessor
2778  definitions.
2779  
2780  As of release 5.6.0, Perl's malloc family of functions have default names
2781  distinct from the system versions.  You need to explicitly compile perl with
2782  C<-DPERL_POLLUTE_MALLOC> to get the older behaviour.  HIDEMYMALLOC
2783  and EMBEDMYMALLOC have no effect, since the behaviour they enabled is now
2784  the default.
2785  
2786  Note that these functions do B<not> constitute Perl's memory allocation API.
2787  See L<perlguts/"Memory Allocation"> for further information about that.
2788  
2789  =back
2790  
2791  =head2 Compatible C Source API Changes
2792  
2793  =over 4
2794  
2795  =item C<PATCHLEVEL> is now C<PERL_VERSION>
2796  
2797  The cpp macros C<PERL_REVISION>, C<PERL_VERSION>, and C<PERL_SUBVERSION>
2798  are now available by default from perl.h, and reflect the base revision,
2799  patchlevel, and subversion respectively.  C<PERL_REVISION> had no
2800  prior equivalent, while C<PERL_VERSION> and C<PERL_SUBVERSION> were
2801  previously available as C<PATCHLEVEL> and C<SUBVERSION>.
2802  
2803  The new names cause less pollution of the B<cpp> namespace and reflect what
2804  the numbers have come to stand for in common practice.  For compatibility,
2805  the old names are still supported when F<patchlevel.h> is explicitly
2806  included (as required before), so there is no source incompatibility
2807  from the change.
2808  
2809  =back
2810  
2811  =head2 Binary Incompatibilities
2812  
2813  In general, the default build of this release is expected to be binary
2814  compatible for extensions built with the 5.005 release or its maintenance
2815  versions.  However, specific platforms may have broken binary compatibility
2816  due to changes in the defaults used in hints files.  Therefore, please be
2817  sure to always check the platform-specific README files for any notes to
2818  the contrary.
2819  
2820  The usethreads or usemultiplicity builds are B<not> binary compatible
2821  with the corresponding builds in 5.005.
2822  
2823  On platforms that require an explicit list of exports (AIX, OS/2 and Windows,
2824  among others), purely internal symbols such as parser functions and the
2825  run time opcodes are not exported by default.  Perl 5.005 used to export
2826  all functions irrespective of whether they were considered part of the
2827  public API or not.
2828  
2829  For the full list of public API functions, see L<perlapi>.
2830  
2831  =head1 Known Problems
2832  
2833  =head2 Thread test failures
2834  
2835  The subtests 19 and 20 of lib/thr5005.t test are known to fail due to
2836  fundamental problems in the 5.005 threading implementation.  These are
2837  not new failures--Perl 5.005_0x has the same bugs, but didn't have these
2838  tests.
2839  
2840  =head2 EBCDIC platforms not supported
2841  
2842  In earlier releases of Perl, EBCDIC environments like OS390 (also
2843  known as Open Edition MVS) and VM-ESA were supported.  Due to changes
2844  required by the UTF-8 (Unicode) support, the EBCDIC platforms are not
2845  supported in Perl 5.6.0.
2846  
2847  =head2 In 64-bit HP-UX the lib/io_multihomed test may hang
2848  
2849  The lib/io_multihomed test may hang in HP-UX if Perl has been
2850  configured to be 64-bit.  Because other 64-bit platforms do not
2851  hang in this test, HP-UX is suspect.  All other tests pass
2852  in 64-bit HP-UX.  The test attempts to create and connect to
2853  "multihomed" sockets (sockets which have multiple IP addresses).
2854  
2855  =head2 NEXTSTEP 3.3 POSIX test failure
2856  
2857  In NEXTSTEP 3.3p2 the implementation of the strftime(3) in the
2858  operating system libraries is buggy: the %j format numbers the days of
2859  a month starting from zero, which, while being logical to programmers,
2860  will cause the subtests 19 to 27 of the lib/posix test may fail.
2861  
2862  =head2 Tru64 (aka Digital UNIX, aka DEC OSF/1) lib/sdbm test failure with gcc
2863  
2864  If compiled with gcc 2.95 the lib/sdbm test will fail (dump core).
2865  The cure is to use the vendor cc, it comes with the operating system
2866  and produces good code.
2867  
2868  =head2 UNICOS/mk CC failures during Configure run
2869  
2870  In UNICOS/mk the following errors may appear during the Configure run:
2871  
2872      Guessing which symbols your C compiler and preprocessor define...
2873      CC-20 cc: ERROR File = try.c, Line = 3
2874      ...
2875        bad switch yylook 79bad switch yylook 79bad switch yylook 79bad switch yylook 79#ifdef A29K
2876      ...
2877      4 errors detected in the compilation of "try.c".
2878  
2879  The culprit is the broken awk of UNICOS/mk.  The effect is fortunately
2880  rather mild: Perl itself is not adversely affected by the error, only
2881  the h2ph utility coming with Perl, and that is rather rarely needed
2882  these days.
2883  
2884  =head2 Arrow operator and arrays
2885  
2886  When the left argument to the arrow operator C<< -> >> is an array, or
2887  the C<scalar> operator operating on an array, the result of the
2888  operation must be considered erroneous. For example:
2889  
2890      @x->[2]
2891      scalar(@x)->[2]
2892  
2893  These expressions will get run-time errors in some future release of
2894  Perl.
2895  
2896  =head2 Experimental features
2897  
2898  As discussed above, many features are still experimental.  Interfaces and
2899  implementation of these features are subject to change, and in extreme cases,
2900  even subject to removal in some future release of Perl.  These features
2901  include the following:
2902  
2903  =over 4
2904  
2905  =item Threads
2906  
2907  =item Unicode
2908  
2909  =item 64-bit support
2910  
2911  =item Lvalue subroutines
2912  
2913  =item Weak references
2914  
2915  =item The pseudo-hash data type
2916  
2917  =item The Compiler suite
2918  
2919  =item Internal implementation of file globbing
2920  
2921  =item The DB module
2922  
2923  =item The regular expression code constructs: 
2924  
2925  C<(?{ code })> and C<(??{ code })>
2926  
2927  =back
2928  
2929  =head1 Obsolete Diagnostics
2930  
2931  =over 4
2932  
2933  =item Character class syntax [: :] is reserved for future extensions
2934  
2935  (W) Within regular expression character classes ([]) the syntax beginning
2936  with "[:" and ending with ":]" is reserved for future extensions.
2937  If you need to represent those character sequences inside a regular
2938  expression character class, just quote the square brackets with the
2939  backslash: "\[:" and ":\]".
2940  
2941  =item Ill-formed logical name |%s| in prime_env_iter
2942  
2943  (W) A warning peculiar to VMS.  A logical name was encountered when preparing
2944  to iterate over %ENV which violates the syntactic rules governing logical
2945  names.  Because it cannot be translated normally, it is skipped, and will not
2946  appear in %ENV.  This may be a benign occurrence, as some software packages
2947  might directly modify logical name tables and introduce nonstandard names,
2948  or it may indicate that a logical name table has been corrupted.
2949  
2950  =item In string, @%s now must be written as \@%s
2951  
2952  The description of this error used to say:
2953  
2954          (Someday it will simply assume that an unbackslashed @
2955           interpolates an array.)
2956  
2957  That day has come, and this fatal error has been removed.  It has been
2958  replaced by a non-fatal warning instead.
2959  See L</Arrays now always interpolate into double-quoted strings> for
2960  details.
2961  
2962  =item Probable precedence problem on %s
2963  
2964  (W) The compiler found a bareword where it expected a conditional,
2965  which often indicates that an || or && was parsed as part of the
2966  last argument of the previous construct, for example:
2967  
2968      open FOO || die;
2969  
2970  =item regexp too big
2971  
2972  (F) The current implementation of regular expressions uses shorts as
2973  address offsets within a string.  Unfortunately this means that if
2974  the regular expression compiles to longer than 32767, it'll blow up.
2975  Usually when you want a regular expression this big, there is a better
2976  way to do it with multiple statements.  See L<perlre>.
2977  
2978  =item Use of "$$<digit>" to mean "${$}<digit>" is deprecated
2979  
2980  (D) Perl versions before 5.004 misinterpreted any type marker followed
2981  by "$" and a digit.  For example, "$$0" was incorrectly taken to mean
2982  "${$}0" instead of "${$0}".  This bug is (mostly) fixed in Perl 5.004.
2983  
2984  However, the developers of Perl 5.004 could not fix this bug completely,
2985  because at least two widely-used modules depend on the old meaning of
2986  "$$0" in a string.  So Perl 5.004 still interprets "$$<digit>" in the
2987  old (broken) way inside strings; but it generates this message as a
2988  warning.  And in Perl 5.005, this special treatment will cease.
2989  
2990  =back
2991  
2992  =head1 Reporting Bugs
2993  
2994  If you find what you think is a bug, you might check the
2995  articles recently posted to the comp.lang.perl.misc newsgroup.
2996  There may also be information at http://www.perl.com/perl/ , the Perl
2997  Home Page.
2998  
2999  If you believe you have an unreported bug, please run the B<perlbug>
3000  program included with your release.  Be sure to trim your bug down
3001  to a tiny but sufficient test case.  Your bug report, along with the
3002  output of C<perl -V>, will be sent off to perlbug@perl.org to be
3003  analysed by the Perl porting team.
3004  
3005  =head1 SEE ALSO
3006  
3007  The F<Changes> file for exhaustive details on what changed.
3008  
3009  The F<INSTALL> file for how to build Perl.
3010  
3011  The F<README> file for general stuff.
3012  
3013  The F<Artistic> and F<Copying> files for copyright information.
3014  
3015  =head1 HISTORY
3016  
3017  Written by Gurusamy Sarathy <F<gsar@activestate.com>>, with many
3018  contributions from The Perl Porters.
3019  
3020  Send omissions or corrections to <F<perlbug@perl.org>>.
3021  
3022  =cut