perl56delta

PERL56DELTA(1)         Perl Programmers Reference Guide         PERL56DELTA(1)

NAME
       perl56delta - what's new for perl v5.6.0

DESCRIPTION
       This document describes differences between the 5.005 release and the
       5.6.0 release.

Core Enhancements
   Interpreter cloning, threads, and concurrency
       Perl 5.6.0 introduces the beginnings of support for running multiple
       interpreters concurrently in different threads.  In conjunction with
       the perl_clone() API call, which can be used to selectively duplicate
       the state of any given interpreter, it is possible to compile a piece
       of code once in an interpreter, clone that interpreter one or more
       times, and run all the resulting interpreters in distinct threads.

       On the Windows platform, this feature is used to emulate fork() at the
       interpreter level.  See perlfork for details about that.

       This feature is still in evolution.  It is eventually meant to be used
       to selectively clone a subroutine and data reachable from that
       subroutine in a separate interpreter and run the cloned subroutine in a
       separate thread.  Since there is no shared data between the
       interpreters, little or no locking will be needed (unless parts of the
       symbol table are explicitly shared).  This is obviously intended to be
       an easy-to-use replacement for the existing threads support.

       Support for cloning interpreters and interpreter concurrency can be
       enabled using the -Dusethreads Configure option (see win32/Makefile for
       how to enable it on Windows.)  The resulting perl executable will be
       functionally identical to one that was built with -Dmultiplicity, but
       the perl_clone() API call will only be available in the former.

       -Dusethreads enables the cpp macro USE_ITHREADS by default, which in
       turn enables Perl source code changes that provide a clear separation
       between the op tree and the data it operates with.  The former is
       immutable, and can therefore be shared between an interpreter and all
       of its clones, while the latter is considered local to each
       interpreter, and is therefore copied for each clone.

       Note that building Perl with the -Dusemultiplicity Configure option is
       adequate if you wish to run multiple independent interpreters
       concurrently in different threads.  -Dusethreads only provides the
       additional functionality of the perl_clone() API call and other support
       for running cloned interpreters concurrently.

           NOTE: This is an experimental feature.  Implementation details are
           subject to change.

   Lexically scoped warning categories
       You can now control the granularity of warnings emitted by perl at a
       finer level using the "use warnings" pragma.  warnings and perllexwarn
       have copious documentation on this feature.

   Unicode and UTF-8 support
       Perl now uses UTF-8 as its internal representation for character
       strings.  The "utf8" and "bytes" pragmas are used to control this
       support in the current lexical scope.  See perlunicode, utf8 and bytes
       for more information.

       This feature is expected to evolve quickly to support some form of I/O
       disciplines that can be used to specify the kind of input and output
       data (bytes or characters).  Until that happens, additional modules
       from CPAN will be needed to complete the toolkit for dealing with
       Unicode.

           NOTE: This should be considered an experimental feature.  Implementation
           details are subject to change.

   Support for interpolating named characters
       The new "\N" escape interpolates named characters within strings.  For
       example, "Hi! \N{WHITE SMILING FACE}" evaluates to a string with a
       unicode smiley face at the end.

   "our" declarations
       An "our" declaration introduces a value that can be best understood as
       a lexically scoped symbolic alias to a global variable in the package
       that was current where the variable was declared.  This is mostly
       useful as an alternative to the "vars" pragma, but also provides the
       opportunity to introduce typing and other attributes for such
       variables.  See "our" in perlfunc.

   Support for strings represented as a vector of ordinals
       Literals of the form "v1.2.3.4" are now parsed as a string composed of
       characters with the specified ordinals.  This is an alternative, more
       readable way to construct (possibly unicode) strings instead of
       interpolating characters, as in "\x{1}\x{2}\x{3}\x{4}".  The leading
       "v" may be omitted if there are more than two ordinals, so 1.2.3 is
       parsed the same as "v1.2.3".

       Strings written in this form are also useful to represent version
       "numbers".  It is easy to compare such version "numbers" (which are
       really just plain strings) using any of the usual string comparison
       operators "eq", "ne", "lt", "gt", etc., or perform bitwise string
       operations on them using "|", "&", etc.

       In conjunction with the new $^V magic variable (which contains the perl
       version as a string), such literals can be used as a readable way to
       check if you're running a particular version of Perl:

           # this will parse in older versions of Perl also
           if ($^V and $^V gt v5.6.0) {
               # new features supported
           }

       "require" and "use" also have some special magic to support such
       literals, but this particular usage should be avoided because it leads
       to misleading error messages under versions of Perl which don't support
       vector strings.  Using a true version number will ensure correct
       behavior in all versions of Perl:

           require 5.006;    # run time check for v5.6
           use 5.006_001;    # compile time check for v5.6.1

       Also, "sprintf" and "printf" support the Perl-specific format flag %v
       to print ordinals of characters in arbitrary strings:

           printf "v%vd", $^V;         # prints current version, such as "v5.5.650"
           printf "%*vX", ":", $addr;  # formats IPv6 address
           printf "%*vb", " ", $bits;  # displays bitstring

       See "Scalar value constructors" in perldata for additional information.

   Improved Perl version numbering system
       Beginning with Perl version 5.6.0, the version number convention has
       been changed to a "dotted integer" scheme that is more commonly found
       in open source projects.

       Maintenance versions of v5.6.0 will be released as v5.6.1, v5.6.2 etc.
       The next development series following v5.6.0 will be numbered v5.7.x,
       beginning with v5.7.0, and the next major production release following
       v5.6.0 will be v5.8.0.

       The English module now sets $PERL_VERSION to $^V (a string value)
       rather than $] (a numeric value).  (This is a potential
       incompatibility.  Send us a report via perlbug if you are affected by
       this.)

       The v1.2.3 syntax is also now legal in Perl.  See "Support for strings
       represented as a vector of ordinals" for more on that.

       To cope with the new versioning system's use of at least three
       significant digits for each version component, the method used for
       incrementing the subversion number has also changed slightly.  We
       assume that versions older than v5.6.0 have been incrementing the
       subversion component in multiples of 10.  Versions after v5.6.0 will
       increment them by 1.  Thus, using the new notation, 5.005_03 is the
       "same" as v5.5.30, and the first maintenance version following v5.6.0
       will be v5.6.1 (which should be read as being equivalent to a floating
       point value of 5.006_001 in the older format, stored in $]).

   New syntax for declaring subroutine attributes
       Formerly, if you wanted to mark a subroutine as being a method call or
       as requiring an automatic lock() when it is entered, you had to declare
       that with a "use attrs" pragma in the body of the subroutine.  That can
       now be accomplished with declaration syntax, like this:

           sub mymethod : locked method;
           ...
           sub mymethod : locked method {
               ...
           }

           sub othermethod :locked :method;
           ...
           sub othermethod :locked :method {
               ...
           }

       (Note how only the first ":" is mandatory, and whitespace surrounding
       the ":" is optional.)

       AutoSplit.pm and SelfLoader.pm have been updated to keep the attributes
       with the stubs they provide.  See attributes.

   File and directory handles can be autovivified
       Similar to how constructs such as "$x->[0]" autovivify a reference,
       handle constructors (open(), opendir(), pipe(), socketpair(),
       sysopen(), socket(), and accept()) now autovivify a file or directory
       handle if the handle passed to them is an uninitialized scalar
       variable.  This allows the constructs such as "open(my $fh, ...)" and
       "open(local $fh,...)"  to be used to create filehandles that will
       conveniently be closed automatically when the scope ends, provided
       there are no other references to them.  This largely eliminates the
       need for typeglobs when opening filehandles that must be passed around,
       as in the following example:

           sub myopen {
               open my $fh, "@_"
                    or die "Can't open '@_': $!";
               return $fh;
           }

           {
               my $f = myopen("</etc/motd");
               print <$f>;
               # $f implicitly closed here
           }

   open() with more than two arguments
       If open() is passed three arguments instead of two, the second argument
       is used as the mode and the third argument is taken to be the file
       name.  This is primarily useful for protecting against unintended magic
       behavior of the traditional two-argument form.  See "open" in perlfunc.

   64-bit support
       Any platform that has 64-bit integers either

               (1) natively as longs or ints
               (2) via special compiler flags
               (3) using long long or int64_t

       is able to use "quads" (64-bit integers) as follows:

       o   constants (decimal, hexadecimal, octal, binary) in the code

       o   arguments to oct() and hex()

       o   arguments to print(), printf() and sprintf() (flag prefixes ll, L,
           q)

       o   printed as such

       o   pack() and unpack() "q" and "Q" formats

       o   in basic arithmetics: + - * / % (NOTE: operating close to the
           limits of the integer values may produce surprising results)

       o   in bit arithmetics: & | ^ ~ << >> (NOTE: these used to be forced to
           be 32 bits wide but now operate on the full native width.)

       o   vec()

       Note that unless you have the case (a) you will have to configure and
       compile Perl using the -Duse64bitint Configure flag.

           NOTE: The Configure flags -Duselonglong and -Duse64bits have been
           deprecated.  Use -Duse64bitint instead.

       There are actually two modes of 64-bitness: the first one is achieved
       using Configure -Duse64bitint and the second one using Configure
       -Duse64bitall.  The difference is that the first one is minimal and the
       second one maximal.  The first works in more places than the second.

       The "use64bitint" does only as much as is required to get 64-bit
       integers into Perl (this may mean, for example, using "long longs")
       while your memory may still be limited to 2 gigabytes (because your
       pointers could still be 32-bit).  Note that the name "64bitint" does
       not imply that your C compiler will be using 64-bit "int"s (it might,
       but it doesn't have to): the "use64bitint" means that you will be able
       to have 64 bits wide scalar values.

       The "use64bitall" goes all the way by attempting to switch also
       integers (if it can), longs (and pointers) to being 64-bit.  This may
       create an even more binary incompatible Perl than -Duse64bitint: the
       resulting executable may not run at all in a 32-bit box, or you may
       have to reboot/reconfigure/rebuild your operating system to be 64-bit
       aware.

       Natively 64-bit systems like Alpha and Cray need neither -Duse64bitint
       nor -Duse64bitall.

       Last but not least: note that due to Perl's habit of always using
       floating point numbers, the quads are still not true integers.  When
       quads overflow their limits (0...18_446_744_073_709_551_615 unsigned,
       -9_223_372_036_854_775_808...9_223_372_036_854_775_807 signed), they
       are silently promoted to floating point numbers, after which they will
       start losing precision (in their lower digits).

           NOTE: 64-bit support is still experimental on most platforms.
           Existing support only covers the LP64 data model.  In particular, the
           LLP64 data model is not yet supported.  64-bit libraries and system
           APIs on many platforms have not stabilized--your mileage may vary.

   Large file support
       If you have filesystems that support "large files" (files larger than 2
       gigabytes), you may now also be able to create and access them from
       Perl.

           NOTE: The default action is to enable large file support, if
           available on the platform.

       If the large file support is on, and you have a Fcntl constant
       O_LARGEFILE, the O_LARGEFILE is automatically added to the flags of
       sysopen().

       Beware that unless your filesystem also supports "sparse files" seeking
       to umpteen petabytes may be inadvisable.

       Note that in addition to requiring a proper file system to do large
       files you may also need to adjust your per-process (or your per-system,
       or per-process-group, or per-user-group) maximum filesize limits before
       running Perl scripts that try to handle large files, especially if you
       intend to write such files.

       Finally, in addition to your process/process group maximum filesize
       limits, you may have quota limits on your filesystems that stop you
       (your user id or your user group id) from using large files.

       Adjusting your process/user/group/file system/operating system limits
       is outside the scope of Perl core language.  For process limits, you
       may try increasing the limits using your shell's limits/limit/ulimit
       command before running Perl.  The BSD::Resource extension (not included
       with the standard Perl distribution) may also be of use, it offers the
       getrlimit/setrlimit interface that can be used to adjust process
       resource usage limits, including the maximum filesize limit.

   Long doubles
       In some systems you may be able to use long doubles to enhance the
       range and precision of your double precision floating point numbers
       (that is, Perl's numbers).  Use Configure -Duselongdouble to enable
       this support (if it is available).

   "more bits"
       You can "Configure -Dusemorebits" to turn on both the 64-bit support
       and the long double support.

   Enhanced support for sort() subroutines
       Perl subroutines with a prototype of "($$)", and XSUBs in general, can
       now be used as sort subroutines.  In either case, the two elements to
       be compared are passed as normal parameters in @_.  See "sort" in
       perlfunc.

       For unprototyped sort subroutines, the historical behavior of passing
       the elements to be compared as the global variables $a and $b remains
       unchanged.

   "sort $coderef @foo" allowed
       sort() did not accept a subroutine reference as the comparison function
       in earlier versions.  This is now permitted.

   File globbing implemented internally
       Perl now uses the File::Glob implementation of the glob() operator
       automatically.  This avoids using an external csh process and the
       problems associated with it.

           NOTE: This is currently an experimental feature.  Interfaces and
           implementation are subject to change.

   Support for CHECK blocks
       In addition to "BEGIN", "INIT", "END", "DESTROY" and "AUTOLOAD",
       subroutines named "CHECK" are now special.  These are queued up during
       compilation and behave similar to END blocks, except they are called at
       the end of compilation rather than at the end of execution.  They
       cannot be called directly.

   POSIX character class syntax [: :] supported
       For example to match alphabetic characters use /[[:alpha:]]/.  See
       perlre for details.

   Better pseudo-random number generator
       In 5.005_0x and earlier, perl's rand() function used the C library
       rand(3) function.  As of 5.005_52, Configure tests for drand48(),
       random(), and rand() (in that order) and picks the first one it finds.

       These changes should result in better random numbers from rand().

   Improved "qw//" operator
       The "qw//" operator is now evaluated at compile time into a true list
       instead of being replaced with a run time call to "split()".  This
       removes the confusing misbehaviour of "qw//" in scalar context, which
       had inherited that behaviour from split().

       Thus:

           $foo = ($bar) = qw(a b c); print "$foo|$bar\n";

       now correctly prints "3|a", instead of "2|a".

   Better worst-case behavior of hashes
       Small changes in the hashing algorithm have been implemented in order
       to improve the distribution of lower order bits in the hashed value.
       This is expected to yield better performance on keys that are repeated
       sequences.

   pack() format 'Z' supported
       The new format type 'Z' is useful for packing and unpacking null-
       terminated strings.  See "pack" in perlfunc.

   pack() format modifier '!' supported
       The new format type modifier '!' is useful for packing and unpacking
       native shorts, ints, and longs.  See "pack" in perlfunc.

   pack() and unpack() support counted strings
       The template character '/' can be used to specify a counted string type
       to be packed or unpacked.  See "pack" in perlfunc.

   Comments in pack() templates
       The '#' character in a template introduces a comment up to end of the
       line.  This facilitates documentation of pack() templates.

   Weak references
       In previous versions of Perl, you couldn't cache objects so as to allow
       them to be deleted if the last reference from outside the cache is
       deleted.  The reference in the cache would hold a reference count on
       the object and the objects would never be destroyed.

       Another familiar problem is with circular references.  When an object
       references itself, its reference count would never go down to zero, and
       it would not get destroyed until the program is about to exit.

       Weak references solve this by allowing you to "weaken" any reference,
       that is, make it not count towards the reference count.  When the last
       non-weak reference to an object is deleted, the object is destroyed and
       all the weak references to the object are automatically undef-ed.

       To use this feature, you need the Devel::WeakRef package from CPAN,
       which contains additional documentation.

           NOTE: This is an experimental feature.  Details are subject to change.

   Binary numbers supported
       Binary numbers are now supported as literals, in s?printf formats, and
       "oct()":

           $answer = 0b101010;
           printf "The answer is: %b\n", oct("0b101010");

   Lvalue subroutines
       Subroutines can now return modifiable lvalues.  See "Lvalue
       subroutines" in perlsub.

           NOTE: This is an experimental feature.  Details are subject to change.

   Some arrows may be omitted in calls through references
       Perl now allows the arrow to be omitted in many constructs involving
       subroutine calls through references.  For example, "$foo[10]->('foo')"
       may now be written "$foo[10]('foo')".  This is rather similar to how
       the arrow may be omitted from "$foo[10]->{'foo'}".  Note however, that
       the arrow is still required for "foo(10)->('bar')".

   Boolean assignment operators are legal lvalues
       Constructs such as "($a ||= 2) += 1" are now allowed.

   exists() is supported on subroutine names
       The exists() builtin now works on subroutine names.  A subroutine is
       considered to exist if it has been declared (even if implicitly).  See
       "exists" in perlfunc for examples.

   exists() and delete() are supported on array elements
       The exists() and delete() builtins now work on simple arrays as well.
       The behavior is similar to that on hash elements.

       exists() can be used to check whether an array element has been
       initialized.  This avoids autovivifying array elements that don't
       exist.  If the array is tied, the EXISTS() method in the corresponding
       tied package will be invoked.

       delete() may be used to remove an element from the array and return it.
       The array element at that position returns to its uninitialized state,
       so that testing for the same element with exists() will return false.
       If the element happens to be the one at the end, the size of the array
       also shrinks up to the highest element that tests true for exists(), or
       0 if none such is found.  If the array is tied, the DELETE() method in
       the corresponding tied package will be invoked.

       See "exists" in perlfunc and "delete" in perlfunc for examples.

   Pseudo-hashes work better
       Dereferencing some types of reference values in a pseudo-hash, such as
       "$ph->{foo}[1]", was accidentally disallowed.  This has been corrected.

       When applied to a pseudo-hash element, exists() now reports whether the
       specified value exists, not merely if the key is valid.

       delete() now works on pseudo-hashes.  When given a pseudo-hash element
       or slice it deletes the values corresponding to the keys (but not the
       keys themselves).  See "Pseudo-hashes: Using an array as a hash" in
       perlref.

       Pseudo-hash slices with constant keys are now optimized to array
       lookups at compile-time.

       List assignments to pseudo-hash slices are now supported.

       The "fields" pragma now provides ways to create pseudo-hashes, via
       fields::new() and fields::phash().  See fields.

           NOTE: The pseudo-hash data type continues to be experimental.
           Limiting oneself to the interface elements provided by the
           fields pragma will provide protection from any future changes.

   Automatic flushing of output buffers
       fork(), exec(), system(), qx//, and pipe open()s now flush buffers of
       all files opened for output when the operation was attempted.  This
       mostly eliminates confusing buffering mishaps suffered by users unaware
       of how Perl internally handles I/O.

       This is not supported on some platforms like Solaris where a suitably
       correct implementation of fflush(NULL) isn't available.

   Better diagnostics on meaningless filehandle operations
       Constructs such as "open(<FH>)" and "close(<FH>)" are compile time
       errors.  Attempting to read from filehandles that were opened only for
       writing will now produce warnings (just as writing to read-only
       filehandles does).

   Where possible, buffered data discarded from duped input filehandle
       "open(NEW, "<&OLD")" now attempts to discard any data that was
       previously read and buffered in "OLD" before duping the handle.  On
       platforms where doing this is allowed, the next read operation on "NEW"
       will return the same data as the corresponding operation on "OLD".
       Formerly, it would have returned the data from the start of the
       following disk block instead.

   eof() has the same old magic as <>
       "eof()" would return true if no attempt to read from "<>" had yet been
       made.  "eof()" has been changed to have a little magic of its own, it
       now opens the "<>" files.

   binmode() can be used to set :crlf and :raw modes
       binmode() now accepts a second argument that specifies a discipline for
       the handle in question.  The two pseudo-disciplines ":raw" and ":crlf"
       are currently supported on DOS-derivative platforms.  See "binmode" in
       perlfunc and open.

   "-T" filetest recognizes UTF-8 encoded files as "text"
       The algorithm used for the "-T" filetest has been enhanced to correctly
       identify UTF-8 content as "text".

   system(), backticks and pipe open now reflect exec() failure
       On Unix and similar platforms, system(), qx() and open(FOO, "cmd |")
       etc., are implemented via fork() and exec().  When the underlying
       exec() fails, earlier versions did not report the error properly, since
       the exec() happened to be in a different process.

       The child process now communicates with the parent about the error in
       launching the external command, which allows these constructs to return
       with their usual error value and set $!.

   Improved diagnostics
       Line numbers are no longer suppressed (under most likely circumstances)
       during the global destruction phase.

       Diagnostics emitted from code running in threads other than the main
       thread are now accompanied by the thread ID.

       Embedded null characters in diagnostics now actually show up.  They
       used to truncate the message in prior versions.

       $foo::a and $foo::b are now exempt from "possible typo" warnings only
       if sort() is encountered in package "foo".

       Unrecognized alphabetic escapes encountered when parsing quote
       constructs now generate a warning, since they may take on new semantics
       in later versions of Perl.

       Many diagnostics now report the internal operation in which the warning
       was provoked, like so:

           Use of uninitialized value in concatenation (.) at (eval 1) line 1.
           Use of uninitialized value in print at (eval 1) line 1.

       Diagnostics  that occur within eval may also report the file and line
       number where the eval is located, in addition to the eval sequence
       number and the line number within the evaluated text itself.  For
       example:

           Not enough arguments for scalar at (eval 4)[newlib/perl5db.pl:1411] line 2, at EOF

   Diagnostics follow STDERR
       Diagnostic output now goes to whichever file the "STDERR" handle is
       pointing at, instead of always going to the underlying C runtime
       library's "stderr".

   More consistent close-on-exec behavior
       On systems that support a close-on-exec flag on filehandles, the flag
       is now set for any handles created by pipe(), socketpair(), socket(),
       and accept(), if that is warranted by the value of $^F that may be in
       effect.  Earlier versions neglected to set the flag for handles created
       with these operators.  See "pipe" in perlfunc, "socketpair" in
       perlfunc, "socket" in perlfunc, "accept" in perlfunc, and "$^F" in
       perlvar.

   syswrite() ease-of-use
       The length argument of "syswrite()" has become optional.

   Better syntax checks on parenthesized unary operators
       Expressions such as:

           print defined(&foo,&bar,&baz);
           print uc("foo","bar","baz");
           undef($foo,&bar);

       used to be accidentally allowed in earlier versions, and produced
       unpredictable behaviour.  Some produced ancillary warnings when used in
       this way; others silently did the wrong thing.

       The parenthesized forms of most unary operators that expect a single
       argument now ensure that they are not called with more than one
       argument, making the cases shown above syntax errors.  The usual
       behaviour of:

           print defined &foo, &bar, &baz;
           print uc "foo", "bar", "baz";
           undef $foo, &bar;

       remains unchanged.  See perlop.

   Bit operators support full native integer width
       The bit operators (& | ^ ~ << >>) now operate on the full native
       integral width (the exact size of which is available in
       $Config{ivsize}).  For example, if your platform is either natively
       64-bit or if Perl has been configured to use 64-bit integers, these
       operations apply to 8 bytes (as opposed to 4 bytes on 32-bit
       platforms).  For portability, be sure to mask off the excess bits in
       the result of unary "~", e.g., "~$x & 0xffffffff".

   Improved security features
       More potentially unsafe operations taint their results for improved
       security.

       The "passwd" and "shell" fields returned by the getpwent(), getpwnam(),
       and getpwuid() are now tainted, because the user can affect their own
       encrypted password and login shell.

       The variable modified by shmread(), and messages returned by msgrcv()
       (and its object-oriented interface IPC::SysV::Msg::rcv) are also
       tainted, because other untrusted processes can modify messages and
       shared memory segments for their own nefarious purposes.

   More functional bareword prototype (*)
       Bareword prototypes have been rationalized to enable them to be used to
       override builtins that accept barewords and interpret them in a special
       way, such as "require" or "do".

       Arguments prototyped as "*" will now be visible within the subroutine
       as either a simple scalar or as a reference to a typeglob.  See
       "Prototypes" in perlsub.

   "require" and "do" may be overridden
       "require" and "do 'file'" operations may be overridden locally by
       importing subroutines of the same name into the current package (or
       globally by importing them into the CORE::GLOBAL:: namespace).
       Overriding "require" will also affect "use", provided the override is
       visible at compile-time.  See "Overriding Built-in Functions" in
       perlsub.

   $^X variables may now have names longer than one character
       Formerly, $^X was synonymous with ${"\cX"}, but $^XY was a syntax
       error.  Now variable names that begin with a control character may be
       arbitrarily long.  However, for compatibility reasons, these variables
       must be written with explicit braces, as "${^XY}" for example.
       "${^XYZ}" is synonymous with ${"\cXYZ"}.  Variable names with more than
       one control character, such as "${^XY^Z}", are illegal.

       The old syntax has not changed.  As before, `^X' may be either a
       literal control-X character or the two-character sequence `caret' plus
       `X'.  When braces are omitted, the variable name stops after the
       control character.  Thus "$^XYZ" continues to be synonymous with "$^X .
       "YZ"" as before.

       As before, lexical variables may not have names beginning with control
       characters.  As before, variables whose names begin with a control
       character are always forced to be in package `main'.  All such
       variables are reserved for future extensions, except those that begin
       with "^_", which may be used by user programs and are guaranteed not to
       acquire special meaning in any future version of Perl.

   New variable $^C reflects "-c" switch
       $^C has a boolean value that reflects whether perl is being run in
       compile-only mode (i.e. via the "-c" switch).  Since BEGIN blocks are
       executed under such conditions, this variable enables perl code to
       determine whether actions that make sense only during normal running
       are warranted.  See perlvar.

   New variable $^V contains Perl version as a string
       $^V contains the Perl version number as a string composed of characters
       whose ordinals match the version numbers, i.e. v5.6.0.  This may be
       used in string comparisons.

       See "Support for strings represented as a vector of ordinals" for an
       example.

   Optional Y2K warnings
       If Perl is built with the cpp macro "PERL_Y2KWARN" defined, it emits
       optional warnings when concatenating the number 19 with another number.

       This behavior must be specifically enabled when running Configure.  See
       INSTALL and README.Y2K.

   Arrays now always interpolate into double-quoted strings
       In double-quoted strings, arrays now interpolate, no matter what.  The
       behavior in earlier versions of perl 5 was that arrays would
       interpolate into strings if the array had been mentioned before the
       string was compiled, and otherwise Perl would raise a fatal compile-
       time error.  In versions 5.000 through 5.003, the error was

               Literal @example now requires backslash

       In versions 5.004_01 through 5.6.0, the error was

               In string, @example now must be written as \@example

       The idea here was to get people into the habit of writing
       "fred\@example.com" when they wanted a literal "@" sign, just as they
       have always written "Give me back my \$5" when they wanted a literal
       "$" sign.

       Starting with 5.6.1, when Perl now sees an "@" sign in a double-quoted
       string, it always attempts to interpolate an array, regardless of
       whether or not the array has been used or declared already.  The fatal
       error has been downgraded to an optional warning:

               Possible unintended interpolation of @example in string

       This warns you that "fred@example.com" is going to turn into "fred.com"
       if you don't backslash the "@".  See
       http://perl.plover.com/at-error.html for more details about the history
       here.

   @- and @+ provide starting/ending offsets of regex matches
       The new magic variables @- and @+ provide the starting and ending
       offsets, respectively, of $&, $1, $2, etc.  See perlvar for details.

Modules and Pragmata
   Modules
       attributes
           While used internally by Perl as a pragma, this module also
           provides a way to fetch subroutine and variable attributes.  See
           attributes.

       B   The Perl Compiler suite has been extensively reworked for this
           release.  More of the standard Perl test suite passes when run
           under the Compiler, but there is still a significant way to go to
           achieve production quality compiled executables.

               NOTE: The Compiler suite remains highly experimental.  The
               generated code may not be correct, even when it manages to execute
               without errors.

       Benchmark
           Overall, Benchmark results exhibit lower average error and better
           timing accuracy.

           You can now run tests for n seconds instead of guessing the right
           number of tests to run: e.g., timethese(-5, ...) will run each code
           for at least 5 CPU seconds.  Zero as the "number of repetitions"
           means "for at least 3 CPU seconds".  The output format has also
           changed.  For example:

              use Benchmark;$x=3;timethese(-5,{a=>sub{$x*$x},b=>sub{$x**2}})

           will now output something like this:

              Benchmark: running a, b, each for at least 5 CPU seconds...
                       a:  5 wallclock secs ( 5.77 usr +  0.00 sys =  5.77 CPU) @ 200551.91/s (n=1156516)
                       b:  4 wallclock secs ( 5.00 usr +  0.02 sys =  5.02 CPU) @ 159605.18/s (n=800686)

           New features: "each for at least N CPU seconds...", "wallclock
           secs", and the "@ operations/CPU second (n=operations)".

           timethese() now returns a reference to a hash of Benchmark objects
           containing the test results, keyed on the names of the tests.

           timethis() now returns the iterations field in the Benchmark result
           object instead of 0.

           timethese(), timethis(), and the new cmpthese() (see below) can
           also take a format specifier of 'none' to suppress output.

           A new function countit() is just like timeit() except that it takes
           a TIME instead of a COUNT.

           A new function cmpthese() prints a chart comparing the results of
           each test returned from a timethese() call.  For each possible pair
           of tests, the percentage speed difference (iters/sec or
           seconds/iter) is shown.

           For other details, see Benchmark.

       ByteLoader
           The ByteLoader is a dedicated extension to generate and run Perl
           bytecode.  See ByteLoader.

       constant
           References can now be used.

           The new version also allows a leading underscore in constant names,
           but disallows a double leading underscore (as in "__LINE__").  Some
           other names are disallowed or warned against, including BEGIN, END,
           etc.  Some names which were forced into main:: used to fail
           silently in some cases; now they're fatal (outside of main::) and
           an optional warning (inside of main::).  The ability to detect
           whether a constant had been set with a given name has been added.

           See constant.

       charnames
           This pragma implements the "\N" string escape.  See charnames.

       Data::Dumper
           A "Maxdepth" setting can be specified to avoid venturing too deeply
           into deep data structures.  See Data::Dumper.

           The XSUB implementation of Dump() is now automatically called if
           the "Useqq" setting is not in use.

           Dumping "qr//" objects works correctly.

       DB  "DB" is an experimental module that exposes a clean abstraction to
           Perl's debugging API.

       DB_File
           DB_File can now be built with Berkeley DB versions 1, 2 or 3.  See
           "ext/DB_File/Changes".

       Devel::DProf
           Devel::DProf, a Perl source code profiler has been added.  See
           Devel::DProf and dprofpp.

       Devel::Peek
           The Devel::Peek module provides access to the internal
           representation of Perl variables and data.  It is a data debugging
           tool for the XS programmer.

       Dumpvalue
           The Dumpvalue module provides screen dumps of Perl data.

       DynaLoader
           DynaLoader now supports a dl_unload_file() function on platforms
           that support unloading shared objects using dlclose().

           Perl can also optionally arrange to unload all extension shared
           objects loaded by Perl.  To enable this, build Perl with the
           Configure option "-Accflags=-DDL_UNLOAD_ALL_AT_EXIT".  (This maybe
           useful if you are using Apache with mod_perl.)

       English
           $PERL_VERSION now stands for $^V (a string value) rather than for
           $] (a numeric value).

       Env Env now supports accessing environment variables like PATH as array
           variables.

       Fcntl
           More Fcntl constants added: F_SETLK64, F_SETLKW64, O_LARGEFILE for
           large file (more than 4GB) access (NOTE: the O_LARGEFILE is
           automatically added to sysopen() flags if large file support has
           been configured, as is the default), Free/Net/OpenBSD locking
           behaviour flags F_FLOCK, F_POSIX, Linux F_SHLCK, and O_ACCMODE: the
           combined mask of O_RDONLY, O_WRONLY, and O_RDWR.  The
           seek()/sysseek() constants SEEK_SET, SEEK_CUR, and SEEK_END are
           available via the ":seek" tag.  The chmod()/stat() S_IF* constants
           and S_IS* functions are available via the ":mode" tag.

       File::Compare
           A compare_text() function has been added, which allows custom
           comparison functions.  See File::Compare.

       File::Find
           File::Find now works correctly when the wanted() function is either
           autoloaded or is a symbolic reference.

           A bug that caused File::Find to lose track of the working directory
           when pruning top-level directories has been fixed.

           File::Find now also supports several other options to control its
           behavior.  It can follow symbolic links if the "follow" option is
           specified.  Enabling the "no_chdir" option will make File::Find
           skip changing the current directory when walking directories.  The
           "untaint" flag can be useful when running with taint checks
           enabled.

           See File::Find.

       File::Glob
           This extension implements BSD-style file globbing.  By default, it
           will also be used for the internal implementation of the glob()
           operator.  See File::Glob.

       File::Spec
           New methods have been added to the File::Spec module: devnull()
           returns the name of the null device (/dev/null on Unix) and
           tmpdir() the name of the temp directory (normally /tmp on Unix).
           There are now also methods to convert between absolute and relative
           filenames: abs2rel() and rel2abs().  For compatibility with
           operating systems that specify volume names in file paths, the
           splitpath(), splitdir(), and catdir() methods have been added.

       File::Spec::Functions
           The new File::Spec::Functions modules provides a function interface
           to the File::Spec module.  Allows shorthand

               $fullname = catfile($dir1, $dir2, $file);

           instead of

               $fullname = File::Spec->catfile($dir1, $dir2, $file);

       Getopt::Long
           Getopt::Long licensing has changed to allow the Perl Artistic
           License as well as the GPL. It used to be GPL only, which got in
           the way of non-GPL applications that wanted to use Getopt::Long.

           Getopt::Long encourages the use of Pod::Usage to produce help
           messages. For example:

               use Getopt::Long;
               use Pod::Usage;
               my $man = 0;
               my $help = 0;
               GetOptions('help|?' => \$help, man => \$man) or pod2usage(2);
               pod2usage(1) if $help;
               pod2usage(-exitstatus => 0, -verbose => 2) if $man;

               __END__

               =head1 NAME

               sample - Using Getopt::Long and Pod::Usage

               =head1 SYNOPSIS

               sample [options] [file ...]

                Options:
                  -help            brief help message
                  -man             full documentation

               =head1 OPTIONS

               =over 8

               =item B<-help>

               Print a brief help message and exits.

               =item B<-man>

               Prints the manual page and exits.

               =back

               =head1 DESCRIPTION

               B<This program> will read the given input file(s) and do something
               useful with the contents thereof.

               =cut

           See Pod::Usage for details.

           A bug that prevented the non-option call-back <> from being
           specified as the first argument has been fixed.

           To specify the characters < and > as option starters, use ><. Note,
           however, that changing option starters is strongly deprecated.

       IO  write() and syswrite() will now accept a single-argument form of
           the call, for consistency with Perl's syswrite().

           You can now create a TCP-based IO::Socket::INET without forcing a
           connect attempt.  This allows you to configure its options (like
           making it non-blocking) and then call connect() manually.

           A bug that prevented the IO::Socket::protocol() accessor from ever
           returning the correct value has been corrected.

           IO::Socket::connect now uses non-blocking IO instead of alarm() to
           do connect timeouts.

           IO::Socket::accept now uses select() instead of alarm() for doing
           timeouts.

           IO::Socket::INET->new now sets $! correctly on failure. $@ is still
           set for backwards compatibility.

       JPL Java Perl Lingo is now distributed with Perl.  See jpl/README for
           more information.

       lib "use lib" now weeds out any trailing duplicate entries.  "no lib"
           removes all named entries.

       Math::BigInt
           The bitwise operations "<<", ">>", "&", "|", and "~" are now
           supported on bigints.

       Math::Complex
           The accessor methods Re, Im, arg, abs, rho, and theta can now also
           act as mutators (accessor $z->Re(), mutator $z->Re(3)).

           The class method "display_format" and the corresponding object
           method "display_format", in addition to accepting just one
           argument, now can also accept a parameter hash.  Recognized keys of
           a parameter hash are "style", which corresponds to the old one
           parameter case, and two new parameters: "format", which is a
           printf()-style format string (defaults usually to "%.15g", you can
           revert to the default by setting the format string to "undef") used
           for both parts of a complex number, and "polar_pretty_print"
           (defaults to true), which controls whether an attempt is made to
           try to recognize small multiples and rationals of pi (2pi, pi/2) at
           the argument (angle) of a polar complex number.

           The potentially disruptive change is that in list context both
           methods now return the parameter hash, instead of only the value of
           the "style" parameter.

       Math::Trig
           A little bit of radial trigonometry (cylindrical and spherical),
           radial coordinate conversions, and the great circle distance were
           added.

       Pod::Parser, Pod::InputObjects
           Pod::Parser is a base class for parsing and selecting sections of
           pod documentation from an input stream.  This module takes care of
           identifying pod paragraphs and commands in the input and hands off
           the parsed paragraphs and commands to user-defined methods which
           are free to interpret or translate them as they see fit.

           Pod::InputObjects defines some input objects needed by Pod::Parser,
           and for advanced users of Pod::Parser that need more about a
           command besides its name and text.

           As of release 5.6.0 of Perl, Pod::Parser is now the officially
           sanctioned "base parser code" recommended for use by all pod2xxx
           translators.  Pod::Text (pod2text) and Pod::Man (pod2man) have
           already been converted to use Pod::Parser and efforts to convert
           Pod::HTML (pod2html) are already underway.  For any questions or
           comments about pod parsing and translating issues and utilities,
           please use the pod-people@perl.org mailing list.

           For further information, please see Pod::Parser and
           Pod::InputObjects.

       Pod::Checker, podchecker
           This utility checks pod files for correct syntax, according to
           perlpod.  Obvious errors are flagged as such, while warnings are
           printed for mistakes that can be handled gracefully.  The checklist
           is not complete yet.  See Pod::Checker.

       Pod::ParseUtils, Pod::Find
           These modules provide a set of gizmos that are useful mainly for
           pod translators.  Pod::Find traverses directory structures and
           returns found pod files, along with their canonical names (like
           "File::Spec::Unix").  Pod::ParseUtils contains Pod::List (useful
           for storing pod list information), Pod::Hyperlink (for parsing the
           contents of "L<>" sequences) and Pod::Cache (for caching
           information about pod files, e.g., link nodes).

       Pod::Select, podselect
           Pod::Select is a subclass of Pod::Parser which provides a function
           named "podselect()" to filter out user-specified sections of raw
           pod documentation from an input stream. podselect is a script that
           provides access to Pod::Select from other scripts to be used as a
           filter.  See Pod::Select.

       Pod::Usage, pod2usage
           Pod::Usage provides the function "pod2usage()" to print usage
           messages for a Perl script based on its embedded pod documentation.
           The pod2usage() function is generally useful to all script authors
           since it lets them write and maintain a single source (the pods)
           for documentation, thus removing the need to create and maintain
           redundant usage message text consisting of information already in
           the pods.

           There is also a pod2usage script which can be used from other kinds
           of scripts to print usage messages from pods (even for non-Perl
           scripts with pods embedded in comments).

           For details and examples, please see Pod::Usage.

       Pod::Text and Pod::Man
           Pod::Text has been rewritten to use Pod::Parser.  While pod2text()
           is still available for backwards compatibility, the module now has
           a new preferred interface.  See Pod::Text for the details.  The new
           Pod::Text module is easily subclassed for tweaks to the output, and
           two such subclasses (Pod::Text::Termcap for man-page-style bold and
           underlining using termcap information, and Pod::Text::Color for
           markup with ANSI color sequences) are now standard.

           pod2man has been turned into a module, Pod::Man, which also uses
           Pod::Parser.  In the process, several outstanding bugs related to
           quotes in section headers, quoting of code escapes, and nested
           lists have been fixed.  pod2man is now a wrapper script around this
           module.

       SDBM_File
           An EXISTS method has been added to this module (and sdbm_exists()
           has been added to the underlying sdbm library), so one can now call
           exists on an SDBM_File tied hash and get the correct result, rather
           than a runtime error.

           A bug that may have caused data loss when more than one disk block
           happens to be read from the database in a single FETCH() has been
           fixed.

       Sys::Syslog
           Sys::Syslog now uses XSUBs to access facilities from syslog.h so it
           no longer requires syslog.ph to exist.

       Sys::Hostname
           Sys::Hostname now uses XSUBs to call the C library's gethostname()
           or uname() if they exist.

       Term::ANSIColor
           Term::ANSIColor is a very simple module to provide easy and
           readable access to the ANSI color and highlighting escape
           sequences, supported by most ANSI terminal emulators.  It is now
           included standard.

       Time::Local
           The timelocal() and timegm() functions used to silently return
           bogus results when the date fell outside the machine's integer
           range.  They now consistently croak() if the date falls in an
           unsupported range.

       Win32
           The error return value in list context has been changed for all
           functions that return a list of values.  Previously these functions
           returned a list with a single element "undef" if an error occurred.
           Now these functions return the empty list in these situations.
           This applies to the following functions:

               Win32::FsType
               Win32::GetOSVersion

           The remaining functions are unchanged and continue to return
           "undef" on error even in list context.

           The Win32::SetLastError(ERROR) function has been added as a
           complement to the Win32::GetLastError() function.

           The new Win32::GetFullPathName(FILENAME) returns the full absolute
           pathname for FILENAME in scalar context.  In list context it
           returns a two-element list containing the fully qualified directory
           name and the filename.  See Win32.

       XSLoader
           The XSLoader extension is a simpler alternative to DynaLoader.  See
           XSLoader.

       DBM Filters
           A new feature called "DBM Filters" has been added to all the DBM
           modules--DB_File, GDBM_File, NDBM_File, ODBM_File, and SDBM_File.
           DBM Filters add four new methods to each DBM module:

               filter_store_key
               filter_store_value
               filter_fetch_key
               filter_fetch_value

           These can be used to filter key-value pairs before the pairs are
           written to the database or just after they are read from the
           database.  See perldbmfilter for further information.

   Pragmata
       "use attrs" is now obsolete, and is only provided for backward-
       compatibility.  It's been replaced by the "sub : attributes" syntax.
       See "Subroutine Attributes" in perlsub and attributes.

       Lexical warnings pragma, "use warnings;", to control optional warnings.
       See perllexwarn.

       "use filetest" to control the behaviour of filetests ("-r" "-w" ...).
       Currently only one subpragma implemented, "use filetest 'access';",
       that uses access(2) or equivalent to check permissions instead of using
       stat(2) as usual.  This matters in filesystems where there are ACLs
       (access control lists): the stat(2) might lie, but access(2) knows
       better.

       The "open" pragma can be used to specify default disciplines for handle
       constructors (e.g. open()) and for qx//.  The two pseudo-disciplines
       ":raw" and ":crlf" are currently supported on DOS-derivative platforms
       (i.e. where binmode is not a no-op).  See also "binmode() can be used
       to set :crlf and :raw modes".

Utility Changes
   dprofpp
       "dprofpp" is used to display profile data generated using
       "Devel::DProf".  See dprofpp.

   find2perl
       The "find2perl" utility now uses the enhanced features of the
       File::Find module.  The -depth and -follow options are supported.  Pod
       documentation is also included in the script.

   h2xs
       The "h2xs" tool can now work in conjunction with "C::Scan" (available
       from CPAN) to automatically parse real-life header files.  The "-M",
       "-a", "-k", and "-o" options are new.

   perlcc
       "perlcc" now supports the C and Bytecode backends.  By default, it
       generates output from the simple C backend rather than the optimized C
       backend.

       Support for non-Unix platforms has been improved.

   perldoc
       "perldoc" has been reworked to avoid possible security holes.  It will
       not by default let itself be run as the superuser, but you may still
       use the -U switch to try to make it drop privileges first.

   The Perl Debugger
       Many bug fixes and enhancements were added to perl5db.pl, the Perl
       debugger.  The help documentation was rearranged.  New commands include
       "< ?", "> ?", and "{ ?" to list out current actions, "man docpage" to
       run your doc viewer on some perl docset, and support for quoted
       options.  The help information was rearranged, and should be viewable
       once again if you're using less as your pager.  A serious security hole
       was plugged--you should immediately remove all older versions of the
       Perl debugger as installed in previous releases, all the way back to
       perl3, from your system to avoid being bitten by this.

Improved Documentation
       Many of the platform-specific README files are now part of the perl
       installation.  See perl for the complete list.

       perlapi.pod
           The official list of public Perl API functions.

       perlboot.pod
           A tutorial for beginners on object-oriented Perl.

       perlcompile.pod
           An introduction to using the Perl Compiler suite.

       perldbmfilter.pod
           A howto document on using the DBM filter facility.

       perldebug.pod
           All material unrelated to running the Perl debugger, plus all low-
           level guts-like details that risked crushing the casual user of the
           debugger, have been relocated from the old manpage to the next
           entry below.

       perldebguts.pod
           This new manpage contains excessively low-level material not
           related to the Perl debugger, but slightly related to debugging
           Perl itself.  It also contains some arcane internal details of how
           the debugging process works that may only be of interest to
           developers of Perl debuggers.

       perlfork.pod
           Notes on the fork() emulation currently available for the Windows
           platform.

       perlfilter.pod
           An introduction to writing Perl source filters.

       perlhack.pod
           Some guidelines for hacking the Perl source code.

       perlintern.pod
           A list of internal functions in the Perl source code.  (List is
           currently empty.)

       perllexwarn.pod
           Introduction and reference information about lexically scoped
           warning categories.

       perlnumber.pod
           Detailed information about numbers as they are represented in Perl.

       perlopentut.pod
           A tutorial on using open() effectively.

       perlreftut.pod
           A tutorial that introduces the essentials of references.

       perltootc.pod
           A tutorial on managing class data for object modules.

       perltodo.pod
           Discussion of the most often wanted features that may someday be
           supported in Perl.

       perlunicode.pod
           An introduction to Unicode support features in Perl.

Performance enhancements
   Simple sort() using { $a <=> $b } and the like are optimized
       Many common sort() operations using a simple inlined block are now
       optimized for faster performance.

   Optimized assignments to lexical variables
       Certain operations in the RHS of assignment statements have been
       optimized to directly set the lexical variable on the LHS, eliminating
       redundant copying overheads.

   Faster subroutine calls
       Minor changes in how subroutine calls are handled internally provide
       marginal improvements in performance.

   delete(), each(), values() and hash iteration are faster
       The hash values returned by delete(), each(), values() and hashes in a
       list context are the actual values in the hash, instead of copies.
       This results in significantly better performance, because it eliminates
       needless copying in most situations.

Installation and Configuration Improvements
   -Dusethreads means something different
       The -Dusethreads flag now enables the experimental interpreter-based
       thread support by default.  To get the flavor of experimental threads
       that was in 5.005 instead, you need to run Configure with "-Dusethreads
       -Duse5005threads".

       As of v5.6.0, interpreter-threads support is still lacking a way to
       create new threads from Perl (i.e., "use Thread;" will not work with
       interpreter threads).  "use Thread;" continues to be available when you
       specify the -Duse5005threads option to Configure, bugs and all.

           NOTE: Support for threads continues to be an experimental feature.
           Interfaces and implementation are subject to sudden and drastic changes.

   New Configure flags
       The following new flags may be enabled on the Configure command line by
       running Configure with "-Dflag".

           usemultiplicity
           usethreads useithreads      (new interpreter threads: no Perl API yet)
           usethreads use5005threads   (threads as they were in 5.005)

           use64bitint                 (equal to now deprecated 'use64bits')
           use64bitall

           uselongdouble
           usemorebits
           uselargefiles
           usesocks                    (only SOCKS v5 supported)

   Threadedness and 64-bitness now more daring
       The Configure options enabling the use of threads and the use of
       64-bitness are now more daring in the sense that they no more have an
       explicit list of operating systems of known threads/64-bit
       capabilities.  In other words: if your operating system has the
       necessary APIs and datatypes, you should be able just to go ahead and
       use them, for threads by Configure -Dusethreads, and for 64 bits either
       explicitly by Configure -Duse64bitint or implicitly if your system has
       64-bit wide datatypes.  See also "64-bit support".

   Long Doubles
       Some platforms have "long doubles", floating point numbers of even
       larger range than ordinary "doubles".  To enable using long doubles for
       Perl's scalars, use -Duselongdouble.

   -Dusemorebits
       You can enable both -Duse64bitint and -Duselongdouble with
       -Dusemorebits.  See also "64-bit support".

   -Duselargefiles
       Some platforms support system APIs that are capable of handling large
       files (typically, files larger than two gigabytes).  Perl will try to
       use these APIs if you ask for -Duselargefiles.

       See "Large file support" for more information.

   installusrbinperl
       You can use "Configure -Uinstallusrbinperl" which causes installperl to
       skip installing perl also as /usr/bin/perl.  This is useful if you
       prefer not to modify /usr/bin for some reason or another but harmful
       because many scripts assume to find Perl in /usr/bin/perl.

   SOCKS support
       You can use "Configure -Dusesocks" which causes Perl to probe for the
       SOCKS proxy protocol library (v5, not v4).  For more information on
       SOCKS, see:

           http://www.socks.nec.com/

   "-A" flag
       You can "post-edit" the Configure variables using the Configure "-A"
       switch.  The editing happens immediately after the platform specific
       hints files have been processed but before the actual configuration
       process starts.  Run "Configure -h" to find out the full "-A" syntax.

   Enhanced Installation Directories
       The installation structure has been enriched to improve the support for
       maintaining multiple versions of perl, to provide locations for vendor-
       supplied modules, scripts, and manpages, and to ease maintenance of
       locally-added modules, scripts, and manpages.  See the section on
       Installation Directories in the INSTALL file for complete details.  For
       most users building and installing from source, the defaults should be
       fine.

       If you previously used "Configure -Dsitelib" or "-Dsitearch" to set
       special values for library directories, you might wish to consider
       using the new "-Dsiteprefix" setting instead.  Also, if you wish to re-
       use a config.sh file from an earlier version of perl, you should be
       sure to check that Configure makes sensible choices for the new
       directories.  See INSTALL for complete details.

Platform specific changes
   Supported platforms
       o   The Mach CThreads (NEXTSTEP, OPENSTEP) are now supported by the
           Thread extension.

       o   GNU/Hurd is now supported.

       o   Rhapsody/Darwin is now supported.

       o   EPOC is now supported (on Psion 5).

       o   The cygwin port (formerly cygwin32) has been greatly improved.

   DOS
       o   Perl now works with djgpp 2.02 (and 2.03 alpha).

       o   Environment variable names are not converted to uppercase any more.

       o   Incorrect exit codes from backticks have been fixed.

       o   This port continues to use its own builtin globbing (not
           File::Glob).

   OS390 (OpenEdition MVS)
       Support for this EBCDIC platform has not been renewed in this release.
       There are difficulties in reconciling Perl's standardization on UTF-8
       as its internal representation for characters with the EBCDIC character
       set, because the two are incompatible.

       It is unclear whether future versions will renew support for this
       platform, but the possibility exists.

   VMS
       Numerous revisions and extensions to configuration, build, testing, and
       installation process to accommodate core changes and VMS-specific
       options.

       Expand %ENV-handling code to allow runtime mapping to logical names,
       CLI symbols, and CRTL environ array.

       Extension of subprocess invocation code to accept filespecs as command
       "verbs".

       Add to Perl command line processing the ability to use default file
       types and to recognize Unix-style "2>&1".

       Expansion of File::Spec::VMS routines, and integration into
       ExtUtils::MM_VMS.

       Extension of ExtUtils::MM_VMS to handle complex extensions more
       flexibly.

       Barewords at start of Unix-syntax paths may be treated as text rather
       than only as logical names.

       Optional secure translation of several logical names used internally by
       Perl.

       Miscellaneous bugfixing and porting of new core code to VMS.

       Thanks are gladly extended to the many people who have contributed VMS
       patches, testing, and ideas.

   Win32
       Perl can now emulate fork() internally, using multiple interpreters
       running in different concurrent threads.  This support must be enabled
       at build time.  See perlfork for detailed information.

       When given a pathname that consists only of a drivename, such as "A:",
       opendir() and stat() now use the current working directory for the
       drive rather than the drive root.

       The builtin XSUB functions in the Win32:: namespace are documented.
       See Win32.

       $^X now contains the full path name of the running executable.

       A Win32::GetLongPathName() function is provided to complement
       Win32::GetFullPathName() and Win32::GetShortPathName().  See Win32.

       POSIX::uname() is supported.

       system(1,...) now returns true process IDs rather than process handles.
       kill() accepts any real process id, rather than strictly return values
       from system(1,...).

       For better compatibility with Unix, "kill(0, $pid)" can now be used to
       test whether a process exists.

       The "Shell" module is supported.

       Better support for building Perl under command.com in Windows 95 has
       been added.

       Scripts are read in binary mode by default to allow ByteLoader (and the
       filter mechanism in general) to work properly.  For compatibility, the
       DATA filehandle will be set to text mode if a carriage return is
       detected at the end of the line containing the __END__ or __DATA__
       token; if not, the DATA filehandle will be left open in binary mode.
       Earlier versions always opened the DATA filehandle in text mode.

       The glob() operator is implemented via the "File::Glob" extension,
       which supports glob syntax of the C shell.  This increases the
       flexibility of the glob() operator, but there may be compatibility
       issues for programs that relied on the older globbing syntax.  If you
       want to preserve compatibility with the older syntax, you might want to
       run perl with "-MFile::DosGlob".  For details and compatibility
       information, see File::Glob.

Significant bug fixes
   <HANDLE> on empty files
       With $/ set to "undef", "slurping" an empty file returns a string of
       zero length (instead of "undef", as it used to) the first time the
       HANDLE is read after $/ is set to "undef".  Further reads yield
       "undef".

       This means that the following will append "foo" to an empty file (it
       used to do nothing):

           perl -0777 -pi -e 's/^/foo/' empty_file

       The behaviour of:

           perl -pi -e 's/^/foo/' empty_file

       is unchanged (it continues to leave the file empty).

   "eval '...'" improvements
       Line numbers (as reflected by caller() and most diagnostics) within
       "eval '...'" were often incorrect where here documents were involved.
       This has been corrected.

       Lexical lookups for variables appearing in "eval '...'" within
       functions that were themselves called within an "eval '...'" were
       searching the wrong place for lexicals.  The lexical search now
       correctly ends at the subroutine's block boundary.

       The use of "return" within "eval {...}" caused $@ not to be reset
       correctly when no exception occurred within the eval.  This has been
       fixed.

       Parsing of here documents used to be flawed when they appeared as the
       replacement expression in "eval 's/.../.../e'".  This has been fixed.

   All compilation errors are true errors
       Some "errors" encountered at compile time were by necessity generated
       as warnings followed by eventual termination of the program.  This
       enabled more such errors to be reported in a single run, rather than
       causing a hard stop at the first error that was encountered.

       The mechanism for reporting such errors has been reimplemented to queue
       compile-time errors and report them at the end of the compilation as
       true errors rather than as warnings.  This fixes cases where error
       messages leaked through in the form of warnings when code was compiled
       at run time using "eval STRING", and also allows such errors to be
       reliably trapped using "eval "..."".

   Implicitly closed filehandles are safer
       Sometimes implicitly closed filehandles (as when they are localized,
       and Perl automatically closes them on exiting the scope) could
       inadvertently set $? or $!.  This has been corrected.

   Behavior of list slices is more consistent
       When taking a slice of a literal list (as opposed to a slice of an
       array or hash), Perl used to return an empty list if the result
       happened to be composed of all undef values.

       The new behavior is to produce an empty list if (and only if) the
       original list was empty.  Consider the following example:

           @a = (1,undef,undef,2)[2,1,2];

       The old behavior would have resulted in @a having no elements.  The new
       behavior ensures it has three undefined elements.

       Note in particular that the behavior of slices of the following cases
       remains unchanged:

           @a = ()[1,2];
           @a = (getpwent)[7,0];
           @a = (anything_returning_empty_list())[2,1,2];
           @a = @b[2,1,2];
           @a = @c{'a','b','c'};

       See perldata.

   "(\$)" prototype and $foo{a}
       A scalar reference prototype now correctly allows a hash or array
       element in that slot.

   "goto &sub" and AUTOLOAD
       The "goto &sub" construct works correctly when &sub happens to be
       autoloaded.

   "-bareword" allowed under "use integer"
       The autoquoting of barewords preceded by "-" did not work in prior
       versions when the "integer" pragma was enabled.  This has been fixed.

   Failures in DESTROY()
       When code in a destructor threw an exception, it went unnoticed in
       earlier versions of Perl, unless someone happened to be looking in $@
       just after the point the destructor happened to run.  Such failures are
       now visible as warnings when warnings are enabled.

   Locale bugs fixed
       printf() and sprintf() previously reset the numeric locale back to the
       default "C" locale.  This has been fixed.

       Numbers formatted according to the local numeric locale (such as using
       a decimal comma instead of a decimal dot) caused "isn't numeric"
       warnings, even while the operations accessing those numbers produced
       correct results.  These warnings have been discontinued.

   Memory leaks
       The "eval 'return sub {...}'" construct could sometimes leak memory.
       This has been fixed.

       Operations that aren't filehandle constructors used to leak memory when
       used on invalid filehandles.  This has been fixed.

       Constructs that modified @_ could fail to deallocate values in @_ and
       thus leak memory.  This has been corrected.

   Spurious subroutine stubs after failed subroutine calls
       Perl could sometimes create empty subroutine stubs when a subroutine
       was not found in the package.  Such cases stopped later method lookups
       from progressing into base packages.  This has been corrected.

   Taint failures under "-U"
       When running in unsafe mode, taint violations could sometimes cause
       silent failures.  This has been fixed.

   END blocks and the "-c" switch
       Prior versions used to run BEGIN and END blocks when Perl was run in
       compile-only mode.  Since this is typically not the expected behavior,
       END blocks are not executed anymore when the "-c" switch is used, or if
       compilation fails.

       See "Support for CHECK blocks" for how to run things when the compile
       phase ends.

   Potential to leak DATA filehandles
       Using the "__DATA__" token creates an implicit filehandle to the file
       that contains the token.  It is the program's responsibility to close
       it when it is done reading from it.

       This caveat is now better explained in the documentation.  See
       perldata.

New or Changed Diagnostics
       "%s" variable %s masks earlier declaration in same %s
           (W misc) A "my" or "our" variable has been redeclared in the
           current scope or statement, effectively eliminating all access to
           the previous instance.  This is almost always a typographical
           error.  Note that the earlier variable will still exist until the
           end of the scope or until all closure referents to it are
           destroyed.

       "my sub" not yet implemented
           (F) Lexically scoped subroutines are not yet implemented.  Don't
           try that yet.

       "our" variable %s redeclared
           (W misc) You seem to have already declared the same global once
           before in the current lexical scope.

       '!' allowed only after types %s
           (F) The '!' is allowed in pack() and unpack() only after certain
           types.  See "pack" in perlfunc.

       / cannot take a count
           (F) You had an unpack template indicating a counted-length string,
           but you have also specified an explicit size for the string.  See
           "pack" in perlfunc.

       / must be followed by a, A or Z
           (F) You had an unpack template indicating a counted-length string,
           which must be followed by one of the letters a, A or Z to indicate
           what sort of string is to be unpacked.  See "pack" in perlfunc.

       / must be followed by a*, A* or Z*
           (F) You had a pack template indicating a counted-length string,
           Currently the only things that can have their length counted are
           a*, A* or Z*.  See "pack" in perlfunc.

       / must follow a numeric type
           (F) You had an unpack template that contained a '#', but this did
           not follow some numeric unpack specification.  See "pack" in
           perlfunc.

       /%s/: Unrecognized escape \\%c passed through
           (W regexp) You used a backslash-character combination which is not
           recognized by Perl.  This combination appears in an interpolated
           variable or a "'"-delimited regular expression.  The character was
           understood literally.

       /%s/: Unrecognized escape \\%c in character class passed through
           (W regexp) You used a backslash-character combination which is not
           recognized by Perl inside character classes.  The character was
           understood literally.

       /%s/ should probably be written as "%s"
           (W syntax) You have used a pattern where Perl expected to find a
           string, as in the first argument to "join".  Perl will treat the
           true or false result of matching the pattern against $_ as the
           string, which is probably not what you had in mind.

       %s() called too early to check prototype
           (W prototype) You've called a function that has a prototype before
           the parser saw a definition or declaration for it, and Perl could
           not check that the call conforms to the prototype.  You need to
           either add an early prototype declaration for the subroutine in
           question, or move the subroutine definition ahead of the call to
           get proper prototype checking.  Alternatively, if you are certain
           that you're calling the function correctly, you may put an
           ampersand before the name to avoid the warning.  See perlsub.

       %s argument is not a HASH or ARRAY element
           (F) The argument to exists() must be a hash or array element, such
           as:

               $foo{$bar}
               $ref->{"susie"}[12]

       %s argument is not a HASH or ARRAY element or slice
           (F) The argument to delete() must be either a hash or array
           element, such as:

               $foo{$bar}
               $ref->{"susie"}[12]

           or a hash or array slice, such as:

               @foo[$bar, $baz, $xyzzy]
               @{$ref->[12]}{"susie", "queue"}

       %s argument is not a subroutine name
           (F) The argument to exists() for "exists &sub" must be a subroutine
           name, and not a subroutine call.  "exists &sub()" will generate
           this error.

       %s package attribute may clash with future reserved word: %s
           (W reserved) A lowercase attribute name was used that had a
           package-specific handler.  That name might have a meaning to Perl
           itself some day, even though it doesn't yet.  Perhaps you should
           use a mixed-case attribute name, instead.  See attributes.

       (in cleanup) %s
           (W misc) This prefix usually indicates that a DESTROY() method
           raised the indicated exception.  Since destructors are usually
           called by the system at arbitrary points during execution, and
           often a vast number of times, the warning is issued only once for
           any number of failures that would otherwise result in the same
           message being repeated.

           Failure of user callbacks dispatched using the "G_KEEPERR" flag
           could also result in this warning.  See "G_KEEPERR" in perlcall.

       <> should be quotes
           (F) You wrote "require <file>" when you should have written
           "require 'file'".

       Attempt to join self
           (F) You tried to join a thread from within itself, which is an
           impossible task.  You may be joining the wrong thread, or you may
           need to move the join() to some other thread.

       Bad evalled substitution pattern
           (F) You've used the /e switch to evaluate the replacement for a
           substitution, but perl found a syntax error in the code to
           evaluate, most likely an unexpected right brace '}'.

       Bad realloc() ignored
           (S) An internal routine called realloc() on something that had
           never been malloc()ed in the first place. Mandatory, but can be
           disabled by setting environment variable "PERL_BADFREE" to 1.

       Bareword found in conditional
           (W bareword) The compiler found a bareword where it expected a
           conditional, which often indicates that an || or && was parsed as
           part of the last argument of the previous construct, for example:

               open FOO || die;

           It may also indicate a misspelled constant that has been
           interpreted as a bareword:

               use constant TYPO => 1;
               if (TYOP) { print "foo" }

           The "strict" pragma is useful in avoiding such errors.

       Binary number > 0b11111111111111111111111111111111 non-portable
           (W portable) The binary number you specified is larger than 2**32-1
           (4294967295) and therefore non-portable between systems.  See
           perlport for more on portability concerns.

       Bit vector size > 32 non-portable
           (W portable) Using bit vector sizes larger than 32 is non-portable.

       Buffer overflow in prime_env_iter: %s
           (W internal) A warning peculiar to VMS.  While Perl was preparing
           to iterate over %ENV, it encountered a logical name or symbol
           definition which was too long, so it was truncated to the string
           shown.

       Can't check filesystem of script "%s"
           (P) For some reason you can't check the filesystem of the script
           for nosuid.

       Can't declare class for non-scalar %s in "%s"
           (S) Currently, only scalar variables can declared with a specific
           class qualifier in a "my" or "our" declaration.  The semantics may
           be extended for other types of variables in future.

       Can't declare %s in "%s"
           (F) Only scalar, array, and hash variables may be declared as "my"
           or "our" variables.  They must have ordinary identifiers as names.

       Can't ignore signal CHLD, forcing to default
           (W signal) Perl has detected that it is being run with the SIGCHLD
           signal (sometimes known as SIGCLD) disabled.  Since disabling this
           signal will interfere with proper determination of exit status of
           child processes, Perl has reset the signal to its default value.
           This situation typically indicates that the parent program under
           which Perl may be running (e.g., cron) is being very careless.

       Can't modify non-lvalue subroutine call
           (F) Subroutines meant to be used in lvalue context should be
           declared as such, see "Lvalue subroutines" in perlsub.

       Can't read CRTL environ
           (S) A warning peculiar to VMS.  Perl tried to read an element of
           %ENV from the CRTL's internal environment array and discovered the
           array was missing.  You need to figure out where your CRTL
           misplaced its environ or define PERL_ENV_TABLES (see perlvms) so
           that environ is not searched.

       Can't remove %s: %s, skipping file
           (S) You requested an inplace edit without creating a backup file.
           Perl was unable to remove the original file to replace it with the
           modified file.  The file was left unmodified.

       Can't return %s from lvalue subroutine
           (F) Perl detected an attempt to return illegal lvalues (such as
           temporary or readonly values) from a subroutine used as an lvalue.
           This is not allowed.

       Can't weaken a nonreference
           (F) You attempted to weaken something that was not a reference.
           Only references can be weakened.

       Character class [:%s:] unknown
           (F) The class in the character class [: :] syntax is unknown.  See
           perlre.

       Character class syntax [%s] belongs inside character classes
           (W unsafe) The character class constructs [: :], [= =], and [. .]
           go inside character classes, the [] are part of the construct, for
           example: /[012[:alpha:]345]/.  Note that [= =] and [. .]  are not
           currently implemented; they are simply placeholders for future
           extensions.

       Constant is not %s reference
           (F) A constant value (perhaps declared using the "use constant"
           pragma) is being dereferenced, but it amounts to the wrong type of
           reference.  The message indicates the type of reference that was
           expected. This usually indicates a syntax error in dereferencing
           the constant value.  See "Constant Functions" in perlsub and
           constant.

       constant(%s): %s
           (F) The parser found inconsistencies either while attempting to
           define an overloaded constant, or when trying to find the character
           name specified in the "\N{...}" escape.  Perhaps you forgot to load
           the corresponding "overload" or "charnames" pragma?  See charnames
           and overload.

       CORE::%s is not a keyword
           (F) The CORE:: namespace is reserved for Perl keywords.

       defined(@array) is deprecated
           (D) defined() is not usually useful on arrays because it checks for
           an undefined scalar value.  If you want to see if the array is
           empty, just use "if (@array) { # not empty }" for example.

       defined(%hash) is deprecated
           (D) defined() is not usually useful on hashes because it checks for
           an undefined scalar value.  If you want to see if the hash is
           empty, just use "if (%hash) { # not empty }" for example.

       Did not produce a valid header
           See Server error.

       (Did you mean "local" instead of "our"?)
           (W misc) Remember that "our" does not localize the declared global
           variable.  You have declared it again in the same lexical scope,
           which seems superfluous.

       Document contains no data
           See Server error.

       entering effective %s failed
           (F) While under the "use filetest" pragma, switching the real and
           effective uids or gids failed.

       false [] range "%s" in regexp
           (W regexp) A character class range must start and end at a literal
           character, not another character class like "\d" or "[:alpha:]".
           The "-" in your false range is interpreted as a literal "-".
           Consider quoting the "-",  "\-".  See perlre.

       Filehandle %s opened only for output
           (W io) You tried to read from a filehandle opened only for writing.
           If you intended it to be a read/write filehandle, you needed to
           open it with "+<" or "+>" or "+>>" instead of with "<" or nothing.
           If you intended only to read from the file, use "<".  See "open" in
           perlfunc.

       flock() on closed filehandle %s
           (W closed) The filehandle you're attempting to flock() got itself
           closed some time before now.  Check your logic flow.  flock()
           operates on filehandles.  Are you attempting to call flock() on a
           dirhandle by the same name?

       Global symbol "%s" requires explicit package name
           (F) You've said "use strict vars", which indicates that all
           variables must either be lexically scoped (using "my"), declared
           beforehand using "our", or explicitly qualified to say which
           package the global variable is in (using "::").

       Hexadecimal number > 0xffffffff non-portable
           (W portable) The hexadecimal number you specified is larger than
           2**32-1 (4294967295) and therefore non-portable between systems.
           See perlport for more on portability concerns.

       Ill-formed CRTL environ value "%s"
           (W internal) A warning peculiar to VMS.  Perl tried to read the
           CRTL's internal environ array, and encountered an element without
           the "=" delimiter used to separate keys from values.  The element
           is ignored.

       Ill-formed message in prime_env_iter: |%s|
           (W internal) A warning peculiar to VMS.  Perl tried to read a
           logical name or CLI symbol definition when preparing to iterate
           over %ENV, and didn't see the expected delimiter between key and
           value, so the line was ignored.

       Illegal binary digit %s
           (F) You used a digit other than 0 or 1 in a binary number.

       Illegal binary digit %s ignored
           (W digit) You may have tried to use a digit other than 0 or 1 in a
           binary number.  Interpretation of the binary number stopped before
           the offending digit.

       Illegal number of bits in vec
           (F) The number of bits in vec() (the third argument) must be a
           power of two from 1 to 32 (or 64, if your platform supports that).

       Integer overflow in %s number
           (W overflow) The hexadecimal, octal or binary number you have
           specified either as a literal or as an argument to hex() or oct()
           is too big for your architecture, and has been converted to a
           floating point number.  On a 32-bit architecture the largest
           hexadecimal, octal or binary number representable without overflow
           is 0xFFFFFFFF, 037777777777, or 0b11111111111111111111111111111111
           respectively.  Note that Perl transparently promotes all numbers to
           a floating point representation internally--subject to loss of
           precision errors in subsequent operations.

       Invalid %s attribute: %s
           The indicated attribute for a subroutine or variable was not
           recognized by Perl or by a user-supplied handler.  See attributes.

       Invalid %s attributes: %s
           The indicated attributes for a subroutine or variable were not
           recognized by Perl or by a user-supplied handler.  See attributes.

       invalid [] range "%s" in regexp
           The offending range is now explicitly displayed.

       Invalid separator character %s in attribute list
           (F) Something other than a colon or whitespace was seen between the
           elements of an attribute list.  If the previous attribute had a
           parenthesised parameter list, perhaps that list was terminated too
           soon.  See attributes.

       Invalid separator character %s in subroutine attribute list
           (F) Something other than a colon or whitespace was seen between the
           elements of a subroutine attribute list.  If the previous attribute
           had a parenthesised parameter list, perhaps that list was
           terminated too soon.

       leaving effective %s failed
           (F) While under the "use filetest" pragma, switching the real and
           effective uids or gids failed.

       Lvalue subs returning %s not implemented yet
           (F) Due to limitations in the current implementation, array and
           hash values cannot be returned in subroutines used in lvalue
           context.  See "Lvalue subroutines" in perlsub.

       Method %s not permitted
           See Server error.

       Missing %sbrace%s on \N{}
           (F) Wrong syntax of character name literal "\N{charname}" within
           double-quotish context.

       Missing command in piped open
           (W pipe) You used the "open(FH, "| command")" or "open(FH, "command
           |")" construction, but the command was missing or blank.

       Missing name in "my sub"
           (F) The reserved syntax for lexically scoped subroutines requires
           that they have a name with which they can be found.

       No %s specified for -%c
           (F) The indicated command line switch needs a mandatory argument,
           but you haven't specified one.

       No package name allowed for variable %s in "our"
           (F) Fully qualified variable names are not allowed in "our"
           declarations, because that doesn't make much sense under existing
           semantics.  Such syntax is reserved for future extensions.

       No space allowed after -%c
           (F) The argument to the indicated command line switch must follow
           immediately after the switch, without intervening spaces.

       no UTC offset information; assuming local time is UTC
           (S) A warning peculiar to VMS.  Perl was unable to find the local
           timezone offset, so it's assuming that local system time is
           equivalent to UTC.  If it's not, define the logical name
           SYS$TIMEZONE_DIFFERENTIAL to translate to the number of seconds
           which need to be added to UTC to get local time.

       Octal number > 037777777777 non-portable
           (W portable) The octal number you specified is larger than 2**32-1
           (4294967295) and therefore non-portable between systems.  See
           perlport for more on portability concerns.

           See also perlport for writing portable code.

       panic: del_backref
           (P) Failed an internal consistency check while trying to reset a
           weak reference.

       panic: kid popen errno read
           (F) forked child returned an incomprehensible message about its
           errno.

       panic: magic_killbackrefs
           (P) Failed an internal consistency check while trying to reset all
           weak references to an object.

       Parentheses missing around "%s" list
           (W parenthesis) You said something like

               my $foo, $bar = @_;

           when you meant

               my ($foo, $bar) = @_;

           Remember that "my", "our", and "local" bind tighter than comma.

       Possible unintended interpolation of %s in string
           (W ambiguous) It used to be that Perl would try to guess whether
           you wanted an array interpolated or a literal @.  It no longer does
           this; arrays are now always interpolated into strings.  This means
           that if you try something like:

                   print "fred@example.com";

           and the array @example doesn't exist, Perl is going to print
           "fred.com", which is probably not what you wanted.  To get a
           literal "@" sign in a string, put a backslash before it, just as
           you would to get a literal "$" sign.

       Possible Y2K bug: %s
           (W y2k) You are concatenating the number 19 with another number,
           which could be a potential Year 2000 problem.

       pragma "attrs" is deprecated, use "sub NAME : ATTRS" instead
           (W deprecated) You have written something like this:

               sub doit
               {
                   use attrs qw(locked);
               }

           You should use the new declaration syntax instead.

               sub doit : locked
               {
                   ...

           The "use attrs" pragma is now obsolete, and is only provided for
           backward-compatibility. See "Subroutine Attributes" in perlsub.

       Premature end of script headers
           See Server error.

       Repeat count in pack overflows
           (F) You can't specify a repeat count so large that it overflows
           your signed integers.  See "pack" in perlfunc.

       Repeat count in unpack overflows
           (F) You can't specify a repeat count so large that it overflows
           your signed integers.  See "unpack" in perlfunc.

       realloc() of freed memory ignored
           (S) An internal routine called realloc() on something that had
           already been freed.

       Reference is already weak
           (W misc) You have attempted to weaken a reference that is already
           weak.  Doing so has no effect.

       setpgrp can't take arguments
           (F) Your system has the setpgrp() from BSD 4.2, which takes no
           arguments, unlike POSIX setpgid(), which takes a process ID and
           process group ID.

       Strange *+?{} on zero-length expression
           (W regexp) You applied a regular expression quantifier in a place
           where it makes no sense, such as on a zero-width assertion.  Try
           putting the quantifier inside the assertion instead.  For example,
           the way to match "abc" provided that it is followed by three
           repetitions of "xyz" is "/abc(?=(?:xyz){3})/", not
           "/abc(?=xyz){3}/".

       switching effective %s is not implemented
           (F) While under the "use filetest" pragma, we cannot switch the
           real and effective uids or gids.

       This Perl can't reset CRTL environ elements (%s)
       This Perl can't set CRTL environ elements (%s=%s)
           (W internal) Warnings peculiar to VMS.  You tried to change or
           delete an element of the CRTL's internal environ array, but your
           copy of Perl wasn't built with a CRTL that contained the setenv()
           function.  You'll need to rebuild Perl with a CRTL that does, or
           redefine PERL_ENV_TABLES (see perlvms) so that the environ array
           isn't the target of the change to %ENV which produced the warning.

       Too late to run %s block
           (W void) A CHECK or INIT block is being defined during run time
           proper, when the opportunity to run them has already passed.
           Perhaps you are loading a file with "require" or "do" when you
           should be using "use" instead.  Or perhaps you should put the
           "require" or "do" inside a BEGIN block.

       Unknown open() mode '%s'
           (F) The second argument of 3-argument open() is not among the list
           of valid modes: "<", ">", ">>", "+<", "+>", "+>>", "-|", "|-".

       Unknown process %x sent message to prime_env_iter: %s
           (P) An error peculiar to VMS.  Perl was reading values for %ENV
           before iterating over it, and someone else stuck a message in the
           stream of data Perl expected.  Someone's very confused, or perhaps
           trying to subvert Perl's population of %ENV for nefarious purposes.

       Unrecognized escape \\%c passed through
           (W misc) You used a backslash-character combination which is not
           recognized by Perl.  The character was understood literally.

       Unterminated attribute parameter in attribute list
           (F) The lexer saw an opening (left) parenthesis character while
           parsing an attribute list, but the matching closing (right)
           parenthesis character was not found.  You may need to add (or
           remove) a backslash character to get your parentheses to balance.
           See attributes.

       Unterminated attribute list
           (F) The lexer found something other than a simple identifier at the
           start of an attribute, and it wasn't a semicolon or the start of a
           block.  Perhaps you terminated the parameter list of the previous
           attribute too soon.  See attributes.

       Unterminated attribute parameter in subroutine attribute list
           (F) The lexer saw an opening (left) parenthesis character while
           parsing a subroutine attribute list, but the matching closing
           (right) parenthesis character was not found.  You may need to add
           (or remove) a backslash character to get your parentheses to
           balance.

       Unterminated subroutine attribute list
           (F) The lexer found something other than a simple identifier at the
           start of a subroutine attribute, and it wasn't a semicolon or the
           start of a block.  Perhaps you terminated the parameter list of the
           previous attribute too soon.

       Value of CLI symbol "%s" too long
           (W misc) A warning peculiar to VMS.  Perl tried to read the value
           of an %ENV element from a CLI symbol table, and found a resultant
           string longer than 1024 characters.  The return value has been
           truncated to 1024 characters.

       Version number must be a constant number
           (P) The attempt to translate a "use Module n.n LIST" statement into
           its equivalent "BEGIN" block found an internal inconsistency with
           the version number.

New tests
       lib/attrs
           Compatibility tests for "sub : attrs" vs the older "use attrs".

       lib/env
           Tests for new environment scalar capability (e.g., "use Env
           qw($BAR);").

       lib/env-array
           Tests for new environment array capability (e.g., "use Env
           qw(@PATH);").

       lib/io_const
           IO constants (SEEK_*, _IO*).

       lib/io_dir
           Directory-related IO methods (new, read, close, rewind, tied
           delete).

       lib/io_multihomed
           INET sockets with multi-homed hosts.

       lib/io_poll
           IO poll().

       lib/io_unix
           UNIX sockets.

       op/attrs
           Regression tests for "my ($x,@y,%z) : attrs" and <sub : attrs>.

       op/filetest
           File test operators.

       op/lex_assign
           Verify operations that access pad objects (lexicals and
           temporaries).

       op/exists_sub
           Verify "exists &sub" operations.

Incompatible Changes
   Perl Source Incompatibilities
       Beware that any new warnings that have been added or old ones that have
       been enhanced are not considered incompatible changes.

       Since all new warnings must be explicitly requested via the "-w" switch
       or the "warnings" pragma, it is ultimately the programmer's
       responsibility to ensure that warnings are enabled judiciously.

       CHECK is a new keyword
           All subroutine definitions named CHECK are now special.  See
           "/"Support for CHECK blocks"" for more information.

       Treatment of list slices of undef has changed
           There is a potential incompatibility in the behavior of list slices
           that are comprised entirely of undefined values.  See "Behavior of
           list slices is more consistent".

       Format of $English::PERL_VERSION is different
           The English module now sets $PERL_VERSION to $^V (a string value)
           rather than $] (a numeric value).  This is a potential
           incompatibility.  Send us a report via perlbug if you are affected
           by this.

           See "Improved Perl version numbering system" for the reasons for
           this change.

       Literals of the form 1.2.3 parse differently
           Previously, numeric literals with more than one dot in them were
           interpreted as a floating point number concatenated with one or
           more numbers.  Such "numbers" are now parsed as strings composed of
           the specified ordinals.

           For example, "print 97.98.99" used to output 97.9899 in earlier
           versions, but now prints "abc".

           See "Support for strings represented as a vector of ordinals".

       Possibly changed pseudo-random number generator
           Perl programs that depend on reproducing a specific set of pseudo-
           random numbers may now produce different output due to improvements
           made to the rand() builtin.  You can use "sh Configure
           -Drandfunc=rand" to obtain the old behavior.

           See "Better pseudo-random number generator".

       Hashing function for hash keys has changed
           Even though Perl hashes are not order preserving, the apparently
           random order encountered when iterating on the contents of a hash
           is actually determined by the hashing algorithm used.  Improvements
           in the algorithm may yield a random order that is different from
           that of previous versions, especially when iterating on hashes.

           See "Better worst-case behavior of hashes" for additional
           information.

       "undef" fails on read only values
           Using the "undef" operator on a readonly value (such as $1) has the
           same effect as assigning "undef" to the readonly value--it throws
           an exception.

       Close-on-exec bit may be set on pipe and socket handles
           Pipe and socket handles are also now subject to the close-on-exec
           behavior determined by the special variable $^F.

           See "More consistent close-on-exec behavior".

       Writing "$$1" to mean "${$}1" is unsupported
           Perl 5.004 deprecated the interpretation of $$1 and similar within
           interpolated strings to mean "$$ . "1"", but still allowed it.

           In Perl 5.6.0 and later, "$$1" always means "${$1}".

       delete(), each(), values() and "\(%h)"
           operate on aliases to values, not copies

           delete(), each(), values() and hashes (e.g. "\(%h)") in a list
           context return the actual values in the hash, instead of copies (as
           they used to in earlier versions).  Typical idioms for using these
           constructs copy the returned values, but this can make a
           significant difference when creating references to the returned
           values.  Keys in the hash are still returned as copies when
           iterating on a hash.

           See also "delete(), each(), values() and hash iteration are
           faster".

       vec(EXPR,OFFSET,BITS) enforces powers-of-two BITS
           vec() generates a run-time error if the BITS argument is not a
           valid power-of-two integer.

       Text of some diagnostic output has changed
           Most references to internal Perl operations in diagnostics have
           been changed to be more descriptive.  This may be an issue for
           programs that may incorrectly rely on the exact text of diagnostics
           for proper functioning.

       "%@" has been removed
           The undocumented special variable "%@" that used to accumulate
           "background" errors (such as those that happen in DESTROY()) has
           been removed, because it could potentially result in memory leaks.

       Parenthesized not() behaves like a list operator
           The "not" operator now falls under the "if it looks like a
           function, it behaves like a function" rule.

           As a result, the parenthesized form can be used with "grep" and
           "map".  The following construct used to be a syntax error before,
           but it works as expected now:

               grep not($_), @things;

           On the other hand, using "not" with a literal list slice may not
           work.  The following previously allowed construct:

               print not (1,2,3)[0];

           needs to be written with additional parentheses now:

               print not((1,2,3)[0]);

           The behavior remains unaffected when "not" is not followed by
           parentheses.

       Semantics of bareword prototype "(*)" have changed
           The semantics of the bareword prototype "*" have changed.  Perl
           5.005 always coerced simple scalar arguments to a typeglob, which
           wasn't useful in situations where the subroutine must distinguish
           between a simple scalar and a typeglob.  The new behavior is to not
           coerce bareword arguments to a typeglob.  The value will always be
           visible as either a simple scalar or as a reference to a typeglob.

           See "More functional bareword prototype (*)".

       Semantics of bit operators may have changed on 64-bit platforms
           If your platform is either natively 64-bit or if Perl has been
           configured to used 64-bit integers, i.e., $Config{ivsize} is 8,
           there may be a potential incompatibility in the behavior of bitwise
           numeric operators (& | ^ ~ << >>).  These operators used to
           strictly operate on the lower 32 bits of integers in previous
           versions, but now operate over the entire native integral width.
           In particular, note that unary "~" will produce different results
           on platforms that have different $Config{ivsize}.  For portability,
           be sure to mask off the excess bits in the result of unary "~",
           e.g., "~$x & 0xffffffff".

           See "Bit operators support full native integer width".

       More builtins taint their results
           As described in "Improved security features", there may be more
           sources of taint in a Perl program.

           To avoid these new tainting behaviors, you can build Perl with the
           Configure option "-Accflags=-DINCOMPLETE_TAINTS".  Beware that the
           ensuing perl binary may be insecure.

   C Source Incompatibilities
       "PERL_POLLUTE"
           Release 5.005 grandfathered old global symbol names by providing
           preprocessor macros for extension source compatibility.  As of
           release 5.6.0, these preprocessor definitions are not available by
           default.  You need to explicitly compile perl with "-DPERL_POLLUTE"
           to get these definitions.  For extensions still using the old
           symbols, this option can be specified via MakeMaker:

               perl Makefile.PL POLLUTE=1

       "PERL_IMPLICIT_CONTEXT"
           This new build option provides a set of macros for all API
           functions such that an implicit interpreter/thread context argument
           is passed to every API function.  As a result of this, something
           like "sv_setsv(foo,bar)" amounts to a macro invocation that
           actually translates to something like
           "Perl_sv_setsv(my_perl,foo,bar)".  While this is generally expected
           to not have any significant source compatibility issues, the
           difference between a macro and a real function call will need to be
           considered.

           This means that there is a source compatibility issue as a result
           of this if your extensions attempt to use pointers to any of the
           Perl API functions.

           Note that the above issue is not relevant to the default build of
           Perl, whose interfaces continue to match those of prior versions
           (but subject to the other options described here).

           See "Background and PERL_IMPLICIT_CONTEXT" in perlguts for detailed
           information on the ramifications of building Perl with this option.

               NOTE: PERL_IMPLICIT_CONTEXT is automatically enabled whenever Perl is built
               with one of -Dusethreads, -Dusemultiplicity, or both.  It is not
               intended to be enabled by users at this time.

       "PERL_POLLUTE_MALLOC"
           Enabling Perl's malloc in release 5.005 and earlier caused the
           namespace of the system's malloc family of functions to be usurped
           by the Perl versions, since by default they used the same names.
           Besides causing problems on platforms that do not allow these
           functions to be cleanly replaced, this also meant that the system
           versions could not be called in programs that used Perl's malloc.
           Previous versions of Perl have allowed this behaviour to be
           suppressed with the HIDEMYMALLOC and EMBEDMYMALLOC preprocessor
           definitions.

           As of release 5.6.0, Perl's malloc family of functions have default
           names distinct from the system versions.  You need to explicitly
           compile perl with "-DPERL_POLLUTE_MALLOC" to get the older
           behaviour.  HIDEMYMALLOC and EMBEDMYMALLOC have no effect, since
           the behaviour they enabled is now the default.

           Note that these functions do not constitute Perl's memory
           allocation API.  See "Memory Allocation" in perlguts for further
           information about that.

   Compatible C Source API Changes
       "PATCHLEVEL" is now "PERL_VERSION"
           The cpp macros "PERL_REVISION", "PERL_VERSION", and
           "PERL_SUBVERSION" are now available by default from perl.h, and
           reflect the base revision, patchlevel, and subversion respectively.
           "PERL_REVISION" had no prior equivalent, while "PERL_VERSION" and
           "PERL_SUBVERSION" were previously available as "PATCHLEVEL" and
           "SUBVERSION".

           The new names cause less pollution of the cpp namespace and reflect
           what the numbers have come to stand for in common practice.  For
           compatibility, the old names are still supported when patchlevel.h
           is explicitly included (as required before), so there is no source
           incompatibility from the change.

   Binary Incompatibilities
       In general, the default build of this release is expected to be binary
       compatible for extensions built with the 5.005 release or its
       maintenance versions.  However, specific platforms may have broken
       binary compatibility due to changes in the defaults used in hints
       files.  Therefore, please be sure to always check the platform-specific
       README files for any notes to the contrary.

       The usethreads or usemultiplicity builds are not binary compatible with
       the corresponding builds in 5.005.

       On platforms that require an explicit list of exports (AIX, OS/2 and
       Windows, among others), purely internal symbols such as parser
       functions and the run time opcodes are not exported by default.  Perl
       5.005 used to export all functions irrespective of whether they were
       considered part of the public API or not.

       For the full list of public API functions, see perlapi.

Known Problems
   Thread test failures
       The subtests 19 and 20 of lib/thr5005.t test are known to fail due to
       fundamental problems in the 5.005 threading implementation.  These are
       not new failures--Perl 5.005_0x has the same bugs, but didn't have
       these tests.

   EBCDIC platforms not supported
       In earlier releases of Perl, EBCDIC environments like OS390 (also known
       as Open Edition MVS) and VM-ESA were supported.  Due to changes
       required by the UTF-8 (Unicode) support, the EBCDIC platforms are not
       supported in Perl 5.6.0.

   In 64-bit HP-UX the lib/io_multihomed test may hang
       The lib/io_multihomed test may hang in HP-UX if Perl has been
       configured to be 64-bit.  Because other 64-bit platforms do not hang in
       this test, HP-UX is suspect.  All other tests pass in 64-bit HP-UX.
       The test attempts to create and connect to "multihomed" sockets
       (sockets which have multiple IP addresses).

   NEXTSTEP 3.3 POSIX test failure
       In NEXTSTEP 3.3p2 the implementation of the strftime(3) in the
       operating system libraries is buggy: the %j format numbers the days of
       a month starting from zero, which, while being logical to programmers,
       will cause the subtests 19 to 27 of the lib/posix test may fail.

   Tru64 (aka Digital UNIX, aka DEC OSF/1) lib/sdbm test failure with gcc
       If compiled with gcc 2.95 the lib/sdbm test will fail (dump core).  The
       cure is to use the vendor cc, it comes with the operating system and
       produces good code.

   UNICOS/mk CC failures during Configure run
       In UNICOS/mk the following errors may appear during the Configure run:

               Guessing which symbols your C compiler and preprocessor define...
               CC-20 cc: ERROR File = try.c, Line = 3
               ...
                 bad switch yylook 79bad switch yylook 79bad switch yylook 79bad switch yylook 79#ifdef A29K
               ...
               4 errors detected in the compilation of "try.c".

       The culprit is the broken awk of UNICOS/mk.  The effect is fortunately
       rather mild: Perl itself is not adversely affected by the error, only
       the h2ph utility coming with Perl, and that is rather rarely needed
       these days.

   Arrow operator and arrays
       When the left argument to the arrow operator "->" is an array, or the
       "scalar" operator operating on an array, the result of the operation
       must be considered erroneous. For example:

           @x->[2]
           scalar(@x)->[2]

       These expressions will get run-time errors in some future release of
       Perl.

   Experimental features
       As discussed above, many features are still experimental.  Interfaces
       and implementation of these features are subject to change, and in
       extreme cases, even subject to removal in some future release of Perl.
       These features include the following:

       Threads
       Unicode
       64-bit support
       Lvalue subroutines
       Weak references
       The pseudo-hash data type
       The Compiler suite
       Internal implementation of file globbing
       The DB module
       The regular expression code constructs:
           "(?{ code })" and "(??{ code })"

Obsolete Diagnostics
       Character class syntax [: :] is reserved for future extensions
           (W) Within regular expression character classes ([]) the syntax
           beginning with "[:" and ending with ":]" is reserved for future
           extensions.  If you need to represent those character sequences
           inside a regular expression character class, just quote the square
           brackets with the backslash: "\[:" and ":\]".

       Ill-formed logical name |%s| in prime_env_iter
           (W) A warning peculiar to VMS.  A logical name was encountered when
           preparing to iterate over %ENV which violates the syntactic rules
           governing logical names.  Because it cannot be translated normally,
           it is skipped, and will not appear in %ENV.  This may be a benign
           occurrence, as some software packages might directly modify logical
           name tables and introduce nonstandard names, or it may indicate
           that a logical name table has been corrupted.

       In string, @%s now must be written as \@%s
           The description of this error used to say:

                   (Someday it will simply assume that an unbackslashed @
                    interpolates an array.)

           That day has come, and this fatal error has been removed.  It has
           been replaced by a non-fatal warning instead.  See "Arrays now
           always interpolate into double-quoted strings" for details.

       Probable precedence problem on %s
           (W) The compiler found a bareword where it expected a conditional,
           which often indicates that an || or && was parsed as part of the
           last argument of the previous construct, for example:

               open FOO || die;

       regexp too big
           (F) The current implementation of regular expressions uses shorts
           as address offsets within a string.  Unfortunately this means that
           if the regular expression compiles to longer than 32767, it'll blow
           up.  Usually when you want a regular expression this big, there is
           a better way to do it with multiple statements.  See perlre.

       Use of "$$<digit>" to mean "${$}<digit>" is deprecated
           (D) Perl versions before 5.004 misinterpreted any type marker
           followed by "$" and a digit.  For example, "$$0" was incorrectly
           taken to mean "${$}0" instead of "${$0}".  This bug is (mostly)
           fixed in Perl 5.004.

           However, the developers of Perl 5.004 could not fix this bug
           completely, because at least two widely-used modules depend on the
           old meaning of "$$0" in a string.  So Perl 5.004 still interprets
           "$$<digit>" in the old (broken) way inside strings; but it
           generates this message as a warning.  And in Perl 5.005, this
           special treatment will cease.

Reporting Bugs
       If you find what you think is a bug, you might check the articles
       recently posted to the comp.lang.perl.misc newsgroup.  There may also
       be information at http://www.perl.com/perl/ , the Perl Home Page.

       If you believe you have an unreported bug, please run the perlbug
       program included with your release.  Be sure to trim your bug down to a
       tiny but sufficient test case.  Your bug report, along with the output
       of "perl -V", will be sent off to perlbug@perl.org to be analysed by
       the Perl porting team.

SEE ALSO
       The Changes file for exhaustive details on what changed.

       The INSTALL file for how to build Perl.

       The README file for general stuff.

       The Artistic and Copying files for copyright information.

HISTORY
       Written by Gurusamy Sarathy <gsar@activestate.com>, with many
       contributions from The Perl Porters.

       Send omissions or corrections to <perlbug@perl.org>.

perl v5.30.0                      2023-11-23                    PERL56DELTA(1)