perl5100delta - Online in the Cloud

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NAME

perl5100delta - what is new for perl 5.10.0

DESCRIPTION

This document describes the differences between the 5.8.8 release and the 5.10.0 release.

Many of the bug fixes in 5.10.0 were already seen in the 5.8.X maintenance releases; they
are not duplicated here and are documented in the set of man pages named
perl58[1-8]?delta.

Core Enhancements

The "feature" pragma
The "feature" pragma is used to enable new syntax that would break Perl's backwards-
compatibility with older releases of the language. It's a lexical pragma, like "strict" or
"warnings".

Currently the following new features are available: "switch" (adds a switch statement),
"say" (adds a "say" built-in function), and "state" (adds a "state" keyword for declaring
"static" variables). Those features are described in their own sections of this document.

The "feature" pragma is also implicitly loaded when you require a minimal perl version
(with the "use VERSION" construct) greater than, or equal to, 5.9.5. See feature for
details.

New -E command-line switch
-E is equivalent to -e, but it implicitly enables all optional features (like "use feature
":5.10"").

Defined-or operator
A new operator "//" (defined-or) has been implemented. The following expression:

$a // $b

is merely equivalent to

defined $a ? $a : $b

and the statement

$c //= $d;

can now be used instead of

$c = $d unless defined $c;

The "//" operator has the same precedence and associativity as "||". Special care has
been taken to ensure that this operator Do What You Mean while not breaking old code, but
some edge cases involving the empty regular expression may now parse differently. See
perlop for details.

Switch and Smart Match operator
Perl 5 now has a switch statement. It's available when "use feature 'switch'" is in
effect. This feature introduces three new keywords, "given", "when", and "default":

given ($foo) {
when (/^abc/) { $abc = 1; }
when (/^def/) { $def = 1; }
when (/^xyz/) { $xyz = 1; }
default { $nothing = 1; }
}

A more complete description of how Perl matches the switch variable against the "when"
conditions is given in "Switch statements" in perlsyn.

This kind of match is called smart match, and it's also possible to use it outside of
switch statements, via the new "~~" operator. See "Smart matching in detail" in perlsyn.

This feature was contributed by Robin Houston.

Regular expressions
Recursive Patterns
It is now possible to write recursive patterns without using the "(??{})" construct.
This new way is more efficient, and in many cases easier to read.

Each capturing parenthesis can now be treated as an independent pattern that can be
entered by using the "(?PARNO)" syntax ("PARNO" standing for "parenthesis number").
For example, the following pattern will match nested balanced angle brackets:

/
^ # start of line
( # start capture buffer 1
< # match an opening angle bracket
(?: # match one of:
(?> # don't backtrack over the inside of this group
[^<>]+ # one or more non angle brackets
) # end non backtracking group
| # ... or ...
(?1) # recurse to bracket 1 and try it again
)* # 0 or more times.
> # match a closing angle bracket
) # end capture buffer one
$ # end of line
/x

PCRE users should note that Perl's recursive regex feature allows backtracking into a
recursed pattern, whereas in PCRE the recursion is atomic or "possessive" in nature.
As in the example above, you can add (?>) to control this selectively. (Yves Orton)

Named Capture Buffers
It is now possible to name capturing parenthesis in a pattern and refer to the
captured contents by name. The naming syntax is "(?<NAME>....)". It's possible to
backreference to a named buffer with the "\k<NAME>" syntax. In code, the new magical
hashes "%+" and "%-" can be used to access the contents of the capture buffers.

Thus, to replace all doubled chars with a single copy, one could write

s/(?<letter>.)\k<letter>/$+{letter}/g

Only buffers with defined contents will be "visible" in the "%+" hash, so it's
possible to do something like

foreach my $name (keys %+) {
print "content of buffer '$name' is $+{$name}\n";
}

The "%-" hash is a bit more complete, since it will contain array refs holding values
from all capture buffers similarly named, if there should be many of them.

"%+" and "%-" are implemented as tied hashes through the new module
"Tie::Hash::NamedCapture".

Users exposed to the .NET regex engine will find that the perl implementation differs
in that the numerical ordering of the buffers is sequential, and not "unnamed first,
then named". Thus in the pattern

/(A)(?<B>B)(C)(?<D>D)/

$1 will be 'A', $2 will be 'B', $3 will be 'C' and $4 will be 'D' and not $1 is 'A',
$2 is 'C' and $3 is 'B' and $4 is 'D' that a .NET programmer would expect. This is
considered a feature. :-) (Yves Orton)

Possessive Quantifiers
Perl now supports the "possessive quantifier" syntax of the "atomic match" pattern.
Basically a possessive quantifier matches as much as it can and never gives any back.
Thus it can be used to control backtracking. The syntax is similar to non-greedy
matching, except instead of using a '?' as the modifier the '+' is used. Thus "?+",
"*+", "++", "{min,max}+" are now legal quantifiers. (Yves Orton)

Backtracking control verbs
The regex engine now supports a number of special-purpose backtrack control verbs:
(*THEN), (*PRUNE), (*MARK), (*SKIP), (*COMMIT), (*FAIL) and (*ACCEPT). See perlre for
their descriptions. (Yves Orton)

Relative backreferences
A new syntax "\g{N}" or "\gN" where "N" is a decimal integer allows a safer form of
back-reference notation as well as allowing relative backreferences. This should make
it easier to generate and embed patterns that contain backreferences. See "Capture
buffers" in perlre. (Yves Orton)

"\K" escape
The functionality of Jeff Pinyan's module Regexp::Keep has been added to the core. In
regular expressions you can now use the special escape "\K" as a way to do something
like floating length positive lookbehind. It is also useful in substitutions like:

s/(foo)bar/$1/g

that can now be converted to

s/foo\Kbar//g

which is much more efficient. (Yves Orton)

Vertical and horizontal whitespace, and linebreak
Regular expressions now recognize the "\v" and "\h" escapes that match vertical and
horizontal whitespace, respectively. "\V" and "\H" logically match their complements.

"\R" matches a generic linebreak, that is, vertical whitespace, plus the multi-
character sequence "\x0D\x0A".

Optional pre-match and post-match captures with the /p flag
There is a new flag "/p" for regular expressions. Using this makes the engine
preserve a copy of the part of the matched string before the matching substring to the
new special variable "${^PREMATCH}", the part after the matching substring to
"${^POSTMATCH}", and the matched substring itself to "${^MATCH}".

Perl is still able to store these substrings to the special variables "$`", "$'", $&,
but using these variables anywhere in the program adds a penalty to all regular
expression matches, whereas if you use the "/p" flag and the new special variables
instead, you pay only for the regular expressions where the flag is used.

For more detail on the new variables, see perlvar; for the use of the regular
expression flag, see perlop and perlre.

"say()"
say() is a new built-in, only available when "use feature 'say'" is in effect, that is
similar to print(), but that implicitly appends a newline to the printed string. See "say"
in perlfunc. (Robin Houston)

Lexical $_
The default variable $_ can now be lexicalized, by declaring it like any other lexical
variable, with a simple

my $_;

The operations that default on $_ will use the lexically-scoped version of $_ when it
exists, instead of the global $_.

In a "map" or a "grep" block, if $_ was previously my'ed, then the $_ inside the block is
lexical as well (and scoped to the block).

In a scope where $_ has been lexicalized, you can still have access to the global version
of $_ by using $::_, or, more simply, by overriding the lexical declaration with "our $_".
(Rafael Garcia-Suarez)

The "_" prototype
A new prototype character has been added. "_" is equivalent to "$" but defaults to $_ if
the corresponding argument isn't supplied (both "$" and "_" denote a scalar). Due to the
optional nature of the argument, you can only use it at the end of a prototype, or before
a semicolon.

This has a small incompatible consequence: the prototype() function has been adjusted to
return "_" for some built-ins in appropriate cases (for example,
"prototype('CORE::rmdir')"). (Rafael Garcia-Suarez)

UNITCHECK blocks
"UNITCHECK", a new special code block has been introduced, in addition to "BEGIN",
"CHECK", "INIT" and "END".

"CHECK" and "INIT" blocks, while useful for some specialized purposes, are always executed
at the transition between the compilation and the execution of the main program, and thus
are useless whenever code is loaded at runtime. On the other hand, "UNITCHECK" blocks are
executed just after the unit which defined them has been compiled. See perlmod for more
information. (Alex Gough)

New Pragma, "mro"
A new pragma, "mro" (for Method Resolution Order) has been added. It permits to switch, on
a per-class basis, the algorithm that perl uses to find inherited methods in case of a
multiple inheritance hierarchy. The default MRO hasn't changed (DFS, for Depth First
Search). Another MRO is available: the C3 algorithm. See mro for more information.
(Brandon Black)

Note that, due to changes in the implementation of class hierarchy search, code that used
to undef the *ISA glob will most probably break. Anyway, undef'ing *ISA had the side-
effect of removing the magic on the @ISA array and should not have been done in the first
place. Also, the cache *::ISA::CACHE:: no longer exists; to force reset the @ISA cache,
you now need to use the "mro" API, or more simply to assign to @ISA (e.g. with "@ISA =
@ISA").

readdir() may return a "short filename" on Windows
The readdir() function may return a "short filename" when the long filename contains
characters outside the ANSI codepage. Similarly Cwd::cwd() may return a short directory
name, and glob() may return short names as well. On the NTFS file system these short
names can always be represented in the ANSI codepage. This will not be true for all other
file system drivers; e.g. the FAT filesystem stores short filenames in the OEM codepage,
so some files on FAT volumes remain unaccessible through the ANSI APIs.

Similarly, $^X, @INC, and $ENV{PATH} are preprocessed at startup to make sure all paths
are valid in the ANSI codepage (if possible).

The Win32::GetLongPathName() function now returns the UTF-8 encoded correct long file name
instead of using replacement characters to force the name into the ANSI codepage. The new
Win32::GetANSIPathName() function can be used to turn a long pathname into a short one
only if the long one cannot be represented in the ANSI codepage.

Many other functions in the "Win32" module have been improved to accept UTF-8 encoded
arguments. Please see Win32 for details.

readpipe() is now overridable
The built-in function readpipe() is now overridable. Overriding it permits also to
override its operator counterpart, "qx//" (a.k.a. "``"). Moreover, it now defaults to $_
if no argument is provided. (Rafael Garcia-Suarez)

Default argument for readline()
readline() now defaults to *ARGV if no argument is provided. (Rafael Garcia-Suarez)

state() variables
A new class of variables has been introduced. State variables are similar to "my"
variables, but are declared with the "state" keyword in place of "my". They're visible
only in their lexical scope, but their value is persistent: unlike "my" variables, they're
not undefined at scope entry, but retain their previous value. (Rafael Garcia-Suarez,
Nicholas Clark)

To use state variables, one needs to enable them by using

use feature 'state';

or by using the "-E" command-line switch in one-liners. See "Persistent Private
Variables" in perlsub.

Stacked filetest operators
As a new form of syntactic sugar, it's now possible to stack up filetest operators. You
can now write "-f -w -x $file" in a row to mean "-x $file && -w _ && -f _". See "-X" in
perlfunc.

UNIVERSAL::DOES()
The "UNIVERSAL" class has a new method, "DOES()". It has been added to solve semantic
problems with the "isa()" method. "isa()" checks for inheritance, while "DOES()" has been
designed to be overridden when module authors use other types of relations between classes
(in addition to inheritance). (chromatic)

See "$obj->DOES( ROLE )" in UNIVERSAL.

Formats
Formats were improved in several ways. A new field, "^*", can be used for variable-width,
one-line-at-a-time text. Null characters are now handled correctly in picture lines. Using
"@#" and "~~" together will now produce a compile-time error, as those format fields are
incompatible. perlform has been improved, and miscellaneous bugs fixed.

Byte-order modifiers for pack() and unpack()
There are two new byte-order modifiers, ">" (big-endian) and "<" (little-endian), that can
be appended to most pack() and unpack() template characters and groups to force a certain
byte-order for that type or group. See "pack" in perlfunc and perlpacktut for details.

"no VERSION"
You can now use "no" followed by a version number to specify that you want to use a
version of perl older than the specified one.

"chdir", "chmod" and "chown" on filehandles
"chdir", "chmod" and "chown" can now work on filehandles as well as filenames, if the
system supports respectively "fchdir", "fchmod" and "fchown", thanks to a patch provided
by Gisle Aas.

OS groups
$( and $) now return groups in the order where the OS returns them, thanks to Gisle Aas.
This wasn't previously the case.

Recursive sort subs
You can now use recursive subroutines with sort(), thanks to Robin Houston.

Exceptions in constant folding
The constant folding routine is now wrapped in an exception handler, and if folding throws
an exception (such as attempting to evaluate 0/0), perl now retains the current optree,
rather than aborting the whole program. Without this change, programs would not compile
if they had expressions that happened to generate exceptions, even though those
expressions were in code that could never be reached at runtime. (Nicholas Clark, Dave
Mitchell)

Source filters in @INC
It's possible to enhance the mechanism of subroutine hooks in @INC by adding a source
filter on top of the filehandle opened and returned by the hook. This feature was planned
a long time ago, but wasn't quite working until now. See "require" in perlfunc for
details. (Nicholas Clark)

New internal variables
"${^RE_DEBUG_FLAGS}"
This variable controls what debug flags are in effect for the regular expression
engine when running under "use re "debug"". See re for details.

"${^CHILD_ERROR_NATIVE}"
This variable gives the native status returned by the last pipe close, backtick
command, successful call to wait() or waitpid(), or from the system() operator. See
perlvar for details. (Contributed by Gisle Aas.)

"${^RE_TRIE_MAXBUF}"
See "Trie optimisation of literal string alternations".

"${^WIN32_SLOPPY_STAT}"
See "Sloppy stat on Windows".

Miscellaneous
"unpack()" now defaults to unpacking the $_ variable.

"mkdir()" without arguments now defaults to $_.

The internal dump output has been improved, so that non-printable characters such as
newline and backspace are output in "\x" notation, rather than octal.

The -C option can no longer be used on the "#!" line. It wasn't working there anyway,
since the standard streams are already set up at this point in the execution of the perl
interpreter. You can use binmode() instead to get the desired behaviour.

UCD 5.0.0
The copy of the Unicode Character Database included in Perl 5 has been updated to version
5.0.0.

MAD
MAD, which stands for Miscellaneous Attribute Decoration, is a still-in-development work
leading to a Perl 5 to Perl 6 converter. To enable it, it's necessary to pass the argument
"-Dmad" to Configure. The obtained perl isn't binary compatible with a regular perl 5.10,
and has space and speed penalties; moreover not all regression tests still pass with it.
(Larry Wall, Nicholas Clark)

kill() on Windows
On Windows platforms, "kill(-9, $pid)" now kills a process tree. (On Unix, this delivers
the signal to all processes in the same process group.)

Incompatible Changes

Packing and UTF-8 strings
The semantics of pack() and unpack() regarding UTF-8-encoded data has been changed.
Processing is now by default character per character instead of byte per byte on the
underlying encoding. Notably, code that used things like "pack("a*", $string)" to see
through the encoding of string will now simply get back the original $string. Packed
strings can also get upgraded during processing when you store upgraded characters. You
can get the old behaviour by using "use bytes".

To be consistent with pack(), the "C0" in unpack() templates indicates that the data is to
be processed in character mode, i.e. character by character; on the contrary, "U0" in
unpack() indicates UTF-8 mode, where the packed string is processed in its UTF-8-encoded
Unicode form on a byte by byte basis. This is reversed with regard to perl 5.8.X, but now
consistent between pack() and unpack().

Moreover, "C0" and "U0" can also be used in pack() templates to specify respectively
character and byte modes.

"C0" and "U0" in the middle of a pack or unpack format now switch to the specified
encoding mode, honoring parens grouping. Previously, parens were ignored.

Also, there is a new pack() character format, "W", which is intended to replace the old
"C". "C" is kept for unsigned chars coded as bytes in the strings internal representation.
"W" represents unsigned (logical) character values, which can be greater than 255. It is
therefore more robust when dealing with potentially UTF-8-encoded data (as "C" will wrap
values outside the range 0..255, and not respect the string encoding).

In practice, that means that pack formats are now encoding-neutral, except "C".

For consistency, "A" in unpack() format now trims all Unicode whitespace from the end of
the string. Before perl 5.9.2, it used to strip only the classical ASCII space characters.

Byte/character count feature in unpack()
A new unpack() template character, ".", returns the number of bytes or characters
(depending on the selected encoding mode, see above) read so far.

The $* and $# variables have been removed
$*, which was deprecated in favor of the "/s" and "/m" regexp modifiers, has been removed.

The deprecated $# variable (output format for numbers) has been removed.

Two new severe warnings, "$#/$* is no longer supported", have been added.

substr() lvalues are no longer fixed-length
The lvalues returned by the three argument form of substr() used to be a "fixed length
window" on the original string. In some cases this could cause surprising action at
distance or other undefined behaviour. Now the length of the window adjusts itself to the
length of the string assigned to it.

Parsing of "-f _"
The identifier "_" is now forced to be a bareword after a filetest operator. This solves a
number of misparsing issues when a global "_" subroutine is defined.

":unique"
The ":unique" attribute has been made a no-op, since its current implementation was
fundamentally flawed and not threadsafe.

Effect of pragmas in eval
The compile-time value of the "%^H" hint variable can now propagate into eval("")uated
code. This makes it more useful to implement lexical pragmas.

As a side-effect of this, the overloaded-ness of constants now propagates into eval("").

chdir FOO
A bareword argument to chdir() is now recognized as a file handle. Earlier releases
interpreted the bareword as a directory name. (Gisle Aas)

Handling of .pmc files
An old feature of perl was that before "require" or "use" look for a file with a .pm
extension, they will first look for a similar filename with a .pmc extension. If this file
is found, it will be loaded in place of any potentially existing file ending in a .pm
extension.

Previously, .pmc files were loaded only if more recent than the matching .pm file.
Starting with 5.9.4, they'll be always loaded if they exist.

$^V is now a "version" object instead of a v-string
$^V can still be used with the %vd format in printf, but any character-level operations
will now access the string representation of the "version" object and not the ordinals of
a v-string. Expressions like "substr($^V, 0, 2)" or "split //, $^V" no longer work and
must be rewritten.

@- and @+ in patterns
The special arrays "@-" and "@+" are no longer interpolated in regular expressions.
(Sadahiro Tomoyuki)

$AUTOLOAD can now be tainted
If you call a subroutine by a tainted name, and if it defers to an AUTOLOAD function, then
$AUTOLOAD will be (correctly) tainted. (Rick Delaney)

Tainting and printf
When perl is run under taint mode, "printf()" and "sprintf()" will now reject any tainted
format argument. (Rafael Garcia-Suarez)

undef and signal handlers
Undefining or deleting a signal handler via "undef $SIG{FOO}" is now equivalent to setting
it to 'DEFAULT'. (Rafael Garcia-Suarez)

strictures and dereferencing in defined()
"use strict 'refs'" was ignoring taking a hard reference in an argument to defined(), as
in :

use strict 'refs';
my $x = 'foo';
if (defined $$x) {...}

This now correctly produces the run-time error "Can't use string as a SCALAR ref while
"strict refs" in use".

"defined @$foo" and "defined %$bar" are now also subject to "strict 'refs'" (that is, $foo
and $bar shall be proper references there.) ("defined(@foo)" and "defined(%bar)" are
discouraged constructs anyway.) (Nicholas Clark)

"(?p{})" has been removed
The regular expression construct "(?p{})", which was deprecated in perl 5.8, has been
removed. Use "(??{})" instead. (Rafael Garcia-Suarez)

Pseudo-hashes have been removed
Support for pseudo-hashes has been removed from Perl 5.9. (The "fields" pragma remains
here, but uses an alternate implementation.)

Removal of the bytecode compiler and of perlcc
"perlcc", the byteloader and the supporting modules (B::C, B::CC, B::Bytecode, etc.) are
no longer distributed with the perl sources. Those experimental tools have never worked
reliably, and, due to the lack of volunteers to keep them in line with the perl
interpreter developments, it was decided to remove them instead of shipping a broken
version of those. The last version of those modules can be found with perl 5.9.4.

However the B compiler framework stays supported in the perl core, as with the more useful
modules it has permitted (among others, B::Deparse and B::Concise).

Removal of the JPL
The JPL (Java-Perl Lingo) has been removed from the perl sources tarball.

Recursive inheritance detected earlier
Perl will now immediately throw an exception if you modify any package's @ISA in such a
way that it would cause recursive inheritance.

Previously, the exception would not occur until Perl attempted to make use of the
recursive inheritance while resolving a method or doing a "$foo->isa($bar)" lookup.

warnings::enabled and warnings::warnif changed to favor users of modules
The behaviour in 5.10.x favors the person using the module; The behaviour in 5.8.x favors
the module writer;

Assume the following code:

main calls Foo::Bar::baz()
Foo::Bar inherits from Foo::Base
Foo::Bar::baz() calls Foo::Base::_bazbaz()
Foo::Base::_bazbaz() calls: warnings::warnif('substr', 'some warning
message');

On 5.8.x, the code warns when Foo::Bar contains "use warnings;" It does not matter if
Foo::Base or main have warnings enabled to disable the warning one has to modify Foo::Bar.

On 5.10.0 and newer, the code warns when main contains "use warnings;" It does not matter
if Foo::Base or Foo::Bar have warnings enabled to disable the warning one has to modify
main.

Modules and Pragmata

Upgrading individual core modules
Even more core modules are now also available separately through the CPAN. If you wish to
update one of these modules, you don't need to wait for a new perl release. From within
the cpan shell, running the 'r' command will report on modules with upgrades available.
See "perldoc CPAN" for more information.

Pragmata Changes
"feature"
The new pragma "feature" is used to enable new features that might break old code. See
"The "feature" pragma" above.

"mro"
This new pragma enables to change the algorithm used to resolve inherited methods. See
"New Pragma, "mro"" above.

Scoping of the "sort" pragma
The "sort" pragma is now lexically scoped. Its effect used to be global.

Scoping of "bignum", "bigint", "bigrat"
The three numeric pragmas "bignum", "bigint" and "bigrat" are now lexically scoped.
(Tels)

"base"
The "base" pragma now warns if a class tries to inherit from itself. (Curtis "Ovid"
Poe)

"strict" and "warnings"
"strict" and "warnings" will now complain loudly if they are loaded via incorrect
casing (as in "use Strict;"). (Johan Vromans)

"version"
The "version" module provides support for version objects.

"warnings"
The "warnings" pragma doesn't load "Carp" anymore. That means that code that used
"Carp" routines without having loaded it at compile time might need to be adjusted;
typically, the following (faulty) code won't work anymore, and will require
parentheses to be added after the function name:

use warnings;
require Carp;
Carp::confess 'argh';

"less"
"less" now does something useful (or at least it tries to). In fact, it has been
turned into a lexical pragma. So, in your modules, you can now test whether your users
have requested to use less CPU, or less memory, less magic, or maybe even less fat.
See less for more. (Joshua ben Jore)

New modules
· "encoding::warnings", by Audrey Tang, is a module to emit warnings whenever an ASCII
character string containing high-bit bytes is implicitly converted into UTF-8. It's a
lexical pragma since Perl 5.9.4; on older perls, its effect is global.

· "Module::CoreList", by Richard Clamp, is a small handy module that tells you what
versions of core modules ship with any versions of Perl 5. It comes with a command-
line frontend, "corelist".

· "Math::BigInt::FastCalc" is an XS-enabled, and thus faster, version of
"Math::BigInt::Calc".

· "Compress::Zlib" is an interface to the zlib compression library. It comes with a
bundled version of zlib, so having a working zlib is not a prerequisite to install it.
It's used by "Archive::Tar" (see below).

· "IO::Zlib" is an "IO::"-style interface to "Compress::Zlib".

· "Archive::Tar" is a module to manipulate "tar" archives.

· "Digest::SHA" is a module used to calculate many types of SHA digests, has been
included for SHA support in the CPAN module.

· "ExtUtils::CBuilder" and "ExtUtils::ParseXS" have been added.

· "Hash::Util::FieldHash", by Anno Siegel, has been added. This module provides support
for field hashes: hashes that maintain an association of a reference with a value, in
a thread-safe garbage-collected way. Such hashes are useful to implement inside-out
objects.

· "Module::Build", by Ken Williams, has been added. It's an alternative to
"ExtUtils::MakeMaker" to build and install perl modules.

· "Module::Load", by Jos Boumans, has been added. It provides a single interface to load
Perl modules and .pl files.

· "Module::Loaded", by Jos Boumans, has been added. It's used to mark modules as loaded
or unloaded.

· "Package::Constants", by Jos Boumans, has been added. It's a simple helper to list all
constants declared in a given package.

· "Win32API::File", by Tye McQueen, has been added (for Windows builds). This module
provides low-level access to Win32 system API calls for files/dirs.

· "Locale::Maketext::Simple", needed by CPANPLUS, is a simple wrapper around
"Locale::Maketext::Lexicon". Note that "Locale::Maketext::Lexicon" isn't included in
the perl core; the behaviour of "Locale::Maketext::Simple" gracefully degrades when
the later isn't present.

· "Params::Check" implements a generic input parsing/checking mechanism. It is used by
CPANPLUS.

· "Term::UI" simplifies the task to ask questions at a terminal prompt.

· "Object::Accessor" provides an interface to create per-object accessors.

· "Module::Pluggable" is a simple framework to create modules that accept pluggable sub-
modules.

· "Module::Load::Conditional" provides simple ways to query and possibly load installed
modules.

· "Time::Piece" provides an object oriented interface to time functions, overriding the
built-ins localtime() and gmtime().

· "IPC::Cmd" helps to find and run external commands, possibly interactively.

· "File::Fetch" provide a simple generic file fetching mechanism.

· "Log::Message" and "Log::Message::Simple" are used by the log facility of "CPANPLUS".

· "Archive::Extract" is a generic archive extraction mechanism for .tar (plain, gzipped
or bzipped) or .zip files.

· "CPANPLUS" provides an API and a command-line tool to access the CPAN mirrors.

· "Pod::Escapes" provides utilities that are useful in decoding Pod E<...> sequences.

· "Pod::Simple" is now the backend for several of the Pod-related modules included with
Perl.

Selected Changes to Core Modules
"Attribute::Handlers"
"Attribute::Handlers" can now report the caller's file and line number. (David
Feldman)

All interpreted attributes are now passed as array references. (Damian Conway)

"B::Lint"
"B::Lint" is now based on "Module::Pluggable", and so can be extended with plugins.
(Joshua ben Jore)

"B" It's now possible to access the lexical pragma hints ("%^H") by using the method
B::COP::hints_hash(). It returns a "B::RHE" object, which in turn can be used to get a
hash reference via the method B::RHE::HASH(). (Joshua ben Jore)

"Thread"
As the old 5005thread threading model has been removed, in favor of the ithreads
scheme, the "Thread" module is now a compatibility wrapper, to be used in old code
only. It has been removed from the default list of dynamic extensions.

Utility Changes

perl -d
The Perl debugger can now save all debugger commands for sourcing later; notably, it
can now emulate stepping backwards, by restarting and rerunning all bar the last
command from a saved command history.

It can also display the parent inheritance tree of a given class, with the "i"
command.

ptar
"ptar" is a pure perl implementation of "tar" that comes with "Archive::Tar".

ptardiff
"ptardiff" is a small utility used to generate a diff between the contents of a tar
archive and a directory tree. Like "ptar", it comes with "Archive::Tar".

shasum
"shasum" is a command-line utility, used to print or to check SHA digests. It comes
with the new "Digest::SHA" module.

corelist
The "corelist" utility is now installed with perl (see "New modules" above).

h2ph and h2xs
"h2ph" and "h2xs" have been made more robust with regard to "modern" C code.

"h2xs" implements a new option "--use-xsloader" to force use of "XSLoader" even in
backwards compatible modules.

The handling of authors' names that had apostrophes has been fixed.

Any enums with negative values are now skipped.

perlivp
"perlivp" no longer checks for *.ph files by default. Use the new "-a" option to run
all tests.

find2perl
"find2perl" now assumes "-print" as a default action. Previously, it needed to be
specified explicitly.

Several bugs have been fixed in "find2perl", regarding "-exec" and "-eval". Also the
options "-path", "-ipath" and "-iname" have been added.

config_data
"config_data" is a new utility that comes with "Module::Build". It provides a command-
line interface to the configuration of Perl modules that use Module::Build's framework
of configurability (that is, *::ConfigData modules that contain local configuration
information for their parent modules.)

cpanp
"cpanp", the CPANPLUS shell, has been added. ("cpanp-run-perl", a helper for CPANPLUS
operation, has been added too, but isn't intended for direct use).

cpan2dist
"cpan2dist" is a new utility that comes with CPANPLUS. It's a tool to create
distributions (or packages) from CPAN modules.

pod2html
The output of "pod2html" has been enhanced to be more customizable via CSS. Some
formatting problems were also corrected. (Jari Aalto)

New Documentation

The perlpragma manpage documents how to write one's own lexical pragmas in pure Perl
(something that is possible starting with 5.9.4).

The new perlglossary manpage is a glossary of terms used in the Perl documentation,
technical and otherwise, kindly provided by O'Reilly Media, Inc.

The perlreguts manpage, courtesy of Yves Orton, describes internals of the Perl regular
expression engine.

The perlreapi manpage describes the interface to the perl interpreter used to write
pluggable regular expression engines (by AEvar Arnfjoer` Bjarmason).

The perlunitut manpage is an tutorial for programming with Unicode and string encodings in
Perl, courtesy of Juerd Waalboer.

A new manual page, perlunifaq (the Perl Unicode FAQ), has been added (Juerd Waalboer).

The perlcommunity manpage gives a description of the Perl community on the Internet and in
real life. (Edgar "Trizor" Bering)

The CORE manual page documents the "CORE::" namespace. (Tels)

The long-existing feature of "/(?{...})/" regexps setting $_ and pos() is now documented.

Performance Enhancements

In-place sorting
Sorting arrays in place ("@a = sort @a") is now optimized to avoid making a temporary copy
of the array.

Likewise, "reverse sort ..." is now optimized to sort in reverse, avoiding the generation
of a temporary intermediate list.

Lexical array access
Access to elements of lexical arrays via a numeric constant between 0 and 255 is now
faster. (This used to be only the case for global arrays.)

XS-assisted SWASHGET
Some pure-perl code that perl was using to retrieve Unicode properties and transliteration
mappings has been reimplemented in XS.

Constant subroutines
The interpreter internals now support a far more memory efficient form of inlineable
constants. Storing a reference to a constant value in a symbol table is equivalent to a
full typeglob referencing a constant subroutine, but using about 400 bytes less memory.
This proxy constant subroutine is automatically upgraded to a real typeglob with
subroutine if necessary. The approach taken is analogous to the existing space
optimisation for subroutine stub declarations, which are stored as plain scalars in place
of the full typeglob.

Several of the core modules have been converted to use this feature for their system
dependent constants - as a result "use POSIX;" now takes about 200K less memory.

"PERL_DONT_CREATE_GVSV"
The new compilation flag "PERL_DONT_CREATE_GVSV", introduced as an option in perl 5.8.8,
is turned on by default in perl 5.9.3. It prevents perl from creating an empty scalar with
every new typeglob. See perl589delta for details.

Weak references are cheaper
Weak reference creation is now O(1) rather than O(n), courtesy of Nicholas Clark. Weak
reference deletion remains O(n), but if deletion only happens at program exit, it may be
skipped completely.

sort() enhancements
Salvador Fandin~o provided improvements to reduce the memory usage of "sort" and to speed
up some cases.

Memory optimisations
Several internal data structures (typeglobs, GVs, CVs, formats) have been restructured to
use less memory. (Nicholas Clark)

UTF-8 cache optimisation
The UTF-8 caching code is now more efficient, and used more often. (Nicholas Clark)

Sloppy stat on Windows
On Windows, perl's stat() function normally opens the file to determine the link count and
update attributes that may have been changed through hard links. Setting
${^WIN32_SLOPPY_STAT} to a true value speeds up stat() by not performing this operation.
(Jan Dubois)

Regular expressions optimisations
Engine de-recursivised
The regular expression engine is no longer recursive, meaning that patterns that used
to overflow the stack will either die with useful explanations, or run to completion,
which, since they were able to blow the stack before, will likely take a very long
time to happen. If you were experiencing the occasional stack overflow (or segfault)
and upgrade to discover that now perl apparently hangs instead, look for a degenerate
regex. (Dave Mitchell)

Single char char-classes treated as literals
Classes of a single character are now treated the same as if the character had been
used as a literal, meaning that code that uses char-classes as an escaping mechanism
will see a speedup. (Yves Orton)

Trie optimisation of literal string alternations
Alternations, where possible, are optimised into more efficient matching structures.
String literal alternations are merged into a trie and are matched simultaneously.
This means that instead of O(N) time for matching N alternations at a given point, the
new code performs in O(1) time. A new special variable, ${^RE_TRIE_MAXBUF}, has been
added to fine-tune this optimization. (Yves Orton)

Note: Much code exists that works around perl's historic poor performance on
alternations. Often the tricks used to do so will disable the new optimisations.
Hopefully the utility modules used for this purpose will be educated about these new
optimisations.

Aho-Corasick start-point optimisation
When a pattern starts with a trie-able alternation and there aren't better
optimisations available, the regex engine will use Aho-Corasick matching to find the
start point. (Yves Orton)

Installation and Configuration Improvements

Configuration improvements
"-Dusesitecustomize"
Run-time customization of @INC can be enabled by passing the "-Dusesitecustomize" flag
to Configure. When enabled, this will make perl run $sitelibexp/sitecustomize.pl
before anything else. This script can then be set up to add additional entries to
@INC.

Relocatable installations
There is now Configure support for creating a relocatable perl tree. If you Configure
with "-Duserelocatableinc", then the paths in @INC (and everything else in %Config)
can be optionally located via the path of the perl executable.

That means that, if the string ".../" is found at the start of any path, it's
substituted with the directory of $^X. So, the relocation can be configured on a per-
directory basis, although the default with "-Duserelocatableinc" is that everything is
relocated. The initial install is done to the original configured prefix.

strlcat() and strlcpy()
The configuration process now detects whether strlcat() and strlcpy() are available.
When they are not available, perl's own version is used (from Russ Allbery's public
domain implementation). Various places in the perl interpreter now use them. (Steve
Peters)

"d_pseudofork" and "d_printf_format_null"
A new configuration variable, available as $Config{d_pseudofork} in the Config module,
has been added, to distinguish real fork() support from fake pseudofork used on
Windows platforms.

A new configuration variable, "d_printf_format_null", has been added, to see if
printf-like formats are allowed to be NULL.

Configure help
"Configure -h" has been extended with the most commonly used options.

Compilation improvements
Parallel build
Parallel makes should work properly now, although there may still be problems if "make
test" is instructed to run in parallel.

Borland's compilers support
Building with Borland's compilers on Win32 should work more smoothly. In particular
Steve Hay has worked to side step many warnings emitted by their compilers and at
least one C compiler internal error.

Static build on Windows
Perl extensions on Windows now can be statically built into the Perl DLL.

Also, it's now possible to build a "perl-static.exe" that doesn't depend on the Perl
DLL on Win32. See the Win32 makefiles for details. (Vadim Konovalov)

ppport.h files
All ppport.h files in the XS modules bundled with perl are now autogenerated at build
time. (Marcus Holland-Moritz)

C++ compatibility
Efforts have been made to make perl and the core XS modules compilable with various
C++ compilers (although the situation is not perfect with some of the compilers on
some of the platforms tested.)

Support for Microsoft 64-bit compiler
Support for building perl with Microsoft's 64-bit compiler has been improved.
(ActiveState)

Visual C++
Perl can now be compiled with Microsoft Visual C++ 2005 (and 2008 Beta 2).

Win32 builds
All win32 builds (MS-Win, WinCE) have been merged and cleaned up.

Installation improvements
Module auxiliary files
README files and changelogs for CPAN modules bundled with perl are no longer
installed.

New Or Improved Platforms
Perl has been reported to work on Symbian OS. See perlsymbian for more information.

Many improvements have been made towards making Perl work correctly on z/OS.

Perl has been reported to work on DragonFlyBSD and MidnightBSD.

Perl has also been reported to work on NexentaOS ( http://www.gnusolaris.org/ ).

The VMS port has been improved. See perlvms.

Support for Cray XT4 Catamount/Qk has been added. See hints/catamount.sh in the source
code distribution for more information.

Vendor patches have been merged for RedHat and Gentoo.

DynaLoader::dl_unload_file() now works on Windows.

Selected Bug Fixes

strictures in regexp-eval blocks
"strict" wasn't in effect in regexp-eval blocks ("/(?{...})/").

Calling CORE::require()
CORE::require() and CORE::do() were always parsed as require() and do() when they were
overridden. This is now fixed.

Subscripts of slices
You can now use a non-arrowed form for chained subscripts after a list slice, like in:

({foo => "bar"})[0]{foo}

This used to be a syntax error; a "->" was required.

"no warnings 'category'" works correctly with -w
Previously when running with warnings enabled globally via "-w", selective disabling
of specific warning categories would actually turn off all warnings. This is now
fixed; now "no warnings 'io';" will only turn off warnings in the "io" class.
Previously it would erroneously turn off all warnings.

threads improvements
Several memory leaks in ithreads were closed. Also, ithreads were made less memory-
intensive.

"threads" is now a dual-life module, also available on CPAN. It has been expanded in
many ways. A kill() method is available for thread signalling. One can get thread
status, or the list of running or joinable threads.

A new "threads->exit()" method is used to exit from the application (this is the
default for the main thread) or from the current thread only (this is the default for
all other threads). On the other hand, the exit() built-in now always causes the whole
application to terminate. (Jerry D. Hedden)

chr() and negative values
chr() on a negative value now gives "\x{FFFD}", the Unicode replacement character,
unless when the "bytes" pragma is in effect, where the low eight bits of the value are
used.

PERL5SHELL and tainting
On Windows, the PERL5SHELL environment variable is now checked for taintedness.
(Rafael Garcia-Suarez)

Using *FILE{IO}
"stat()" and "-X" filetests now treat *FILE{IO} filehandles like *FILE filehandles.
(Steve Peters)

Overloading and reblessing
Overloading now works when references are reblessed into another class. Internally,
this has been implemented by moving the flag for "overloading" from the reference to
the referent, which logically is where it should always have been. (Nicholas Clark)

Overloading and UTF-8
A few bugs related to UTF-8 handling with objects that have stringification overloaded
have been fixed. (Nicholas Clark)

eval memory leaks fixed
Traditionally, "eval 'syntax error'" has leaked badly. Many (but not all) of these
leaks have now been eliminated or reduced. (Dave Mitchell)

Random device on Windows
In previous versions, perl would read the file /dev/urandom if it existed when seeding
its random number generator. That file is unlikely to exist on Windows, and if it did
would probably not contain appropriate data, so perl no longer tries to read it on
Windows. (Alex Davies)

PERLIO_DEBUG
The "PERLIO_DEBUG" environment variable no longer has any effect for setuid scripts
and for scripts run with -T.

Moreover, with a thread-enabled perl, using "PERLIO_DEBUG" could lead to an internal
buffer overflow. This has been fixed.

PerlIO::scalar and read-only scalars
PerlIO::scalar will now prevent writing to read-only scalars. Moreover, seek() is now
supported with PerlIO::scalar-based filehandles, the underlying string being zero-
filled as needed. (Rafael, Jarkko Hietaniemi)

study() and UTF-8
study() never worked for UTF-8 strings, but could lead to false results. It's now a
no-op on UTF-8 data. (Yves Orton)

Critical signals
The signals SIGILL, SIGBUS and SIGSEGV are now always delivered in an "unsafe" manner
(contrary to other signals, that are deferred until the perl interpreter reaches a
reasonably stable state; see "Deferred Signals (Safe Signals)" in perlipc). (Rafael)

@INC-hook fix
When a module or a file is loaded through an @INC-hook, and when this hook has set a
filename entry in %INC, __FILE__ is now set for this module accordingly to the
contents of that %INC entry. (Rafael)

"-t" switch fix
The "-w" and "-t" switches can now be used together without messing up which
categories of warnings are activated. (Rafael)

Duping UTF-8 filehandles
Duping a filehandle which has the ":utf8" PerlIO layer set will now properly carry
that layer on the duped filehandle. (Rafael)

Localisation of hash elements
Localizing a hash element whose key was given as a variable didn't work correctly if
the variable was changed while the local() was in effect (as in "local $h{$x}; ++$x").
(Bo Lindbergh)

New or Changed Diagnostics

Use of uninitialized value
Perl will now try to tell you the name of the variable (if any) that was undefined.

Deprecated use of my() in false conditional
A new deprecation warning, Deprecated use of my() in false conditional, has been
added, to warn against the use of the dubious and deprecated construct

my $x if 0;

See perldiag. Use "state" variables instead.

!=~ should be !~
A new warning, "!=~ should be !~", is emitted to prevent this misspelling of the non-
matching operator.

Newline in left-justified string
The warning Newline in left-justified string has been removed.

Too late for "-T" option
The error Too late for "-T" option has been reformulated to be more descriptive.

"%s" variable %s masks earlier declaration
This warning is now emitted in more consistent cases; in short, when one of the
declarations involved is a "my" variable:

my $x; my $x; # warns
my $x; our $x; # warns
our $x; my $x; # warns

On the other hand, the following:

our $x; our $x;

now gives a ""our" variable %s redeclared" warning.

readdir()/closedir()/etc. attempted on invalid dirhandle
These new warnings are now emitted when a dirhandle is used but is either closed or
not really a dirhandle.

Opening dirhandle/filehandle %s also as a file/directory
Two deprecation warnings have been added: (Rafael)

Opening dirhandle %s also as a file
Opening filehandle %s also as a directory

Use of -P is deprecated
Perl's command-line switch "-P" is now deprecated.

v-string in use/require is non-portable
Perl will warn you against potential backwards compatibility problems with the "use
VERSION" syntax.

perl -V
"perl -V" has several improvements, making it more useable from shell scripts to get
the value of configuration variables. See perlrun for details.

Changed Internals

In general, the source code of perl has been refactored, tidied up, and optimized in many
places. Also, memory management and allocation has been improved in several points.

When compiling the perl core with gcc, as many gcc warning flags are turned on as is
possible on the platform. (This quest for cleanliness doesn't extend to XS code because
we cannot guarantee the tidiness of code we didn't write.) Similar strictness flags have
been added or tightened for various other C compilers.

Reordering of SVt_* constants
The relative ordering of constants that define the various types of "SV" have changed; in
particular, "SVt_PVGV" has been moved before "SVt_PVLV", "SVt_PVAV", "SVt_PVHV" and
"SVt_PVCV". This is unlikely to make any difference unless you have code that explicitly
makes assumptions about that ordering. (The inheritance hierarchy of "B::*" objects has
been changed to reflect this.)

Elimination of SVt_PVBM
Related to this, the internal type "SVt_PVBM" has been removed. This dedicated type of
"SV" was used by the "index" operator and parts of the regexp engine to facilitate fast
Boyer-Moore matches. Its use internally has been replaced by "SV"s of type "SVt_PVGV".

New type SVt_BIND
A new type "SVt_BIND" has been added, in readiness for the project to implement Perl 6 on
5. There deliberately is no implementation yet, and they cannot yet be created or
destroyed.

Removal of CPP symbols
The C preprocessor symbols "PERL_PM_APIVERSION" and "PERL_XS_APIVERSION", which were
supposed to give the version number of the oldest perl binary-compatible (resp. source-
compatible) with the present one, were not used, and sometimes had misleading values. They
have been removed.

Less space is used by ops
The "BASEOP" structure now uses less space. The "op_seq" field has been removed and
replaced by a single bit bit-field "op_opt". "op_type" is now 9 bits long. (Consequently,
the "B::OP" class doesn't provide an "seq" method anymore.)

New parser
perl's parser is now generated by bison (it used to be generated by byacc.) As a result,
it seems to be a bit more robust.

Also, Dave Mitchell improved the lexer debugging output under "-DT".

Use of "const"
Andy Lester supplied many improvements to determine which function parameters and local
variables could actually be declared "const" to the C compiler. Steve Peters provided new
*_set macros and reworked the core to use these rather than assigning to macros in LVALUE
context.

Mathoms
A new file, mathoms.c, has been added. It contains functions that are no longer used in
the perl core, but that remain available for binary or source compatibility reasons.
However, those functions will not be compiled in if you add "-DNO_MATHOMS" in the compiler
flags.

"AvFLAGS" has been removed
The "AvFLAGS" macro has been removed.

"av_*" changes
The "av_*()" functions, used to manipulate arrays, no longer accept null "AV*" parameters.

$^H and %^H
The implementation of the special variables $^H and %^H has changed, to allow implementing
lexical pragmas in pure Perl.

B:: modules inheritance changed
The inheritance hierarchy of "B::" modules has changed; "B::NV" now inherits from "B::SV"
(it used to inherit from "B::IV").

Anonymous hash and array constructors
The anonymous hash and array constructors now take 1 op in the optree instead of 3, now
that pp_anonhash and pp_anonlist return a reference to an hash/array when the op is
flagged with OPf_SPECIAL. (Nicholas Clark)

Known Problems

There's still a remaining problem in the implementation of the lexical $_: it doesn't work
inside "/(?{...})/" blocks. (See the TODO test in t/op/mydef.t.)

Stacked filetest operators won't work when the "filetest" pragma is in effect, because
they rely on the stat() buffer "_" being populated, and filetest bypasses stat().

UTF-8 problems
The handling of Unicode still is unclean in several places, where it's dependent on
whether a string is internally flagged as UTF-8. This will be made more consistent in perl
5.12, but that won't be possible without a certain amount of backwards incompatibility.

Platform Specific Problems

When compiled with g++ and thread support on Linux, it's reported that the $! stops
working correctly. This is related to the fact that the glibc provides two strerror_r(3)
implementation, and perl selects the wrong one.

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 and the perl bug database at http://rt.perl.org/rt3/ .
There may also be information at http://www.perl.org/ , 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 [email protected] to be
analysed by the Perl porting team.

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