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=head1 NAME

perl594delta - what is new for perl v5.9.4

=head1 DESCRIPTION

This document describes differences between the 5.9.3 and the 5.9.4
development releases. See L<perl590delta>, L<perl591delta>, L<perl592delta>
and L<perl593delta> for the differences between 5.8.0 and 5.9.3.

=head1 Incompatible Changes

=head2 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)

=head2 Handling of pmc files

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

Previously, F<.pmc> files were loaded only if more recent than the
matching F<.pm> file. Starting with 5.9.4, they'll be always loaded if
they exist. (This trick is used by Pugs.)

=head2 @- and @+ in patterns

The special arrays C<@-> and C<@+> are no longer interpolated in regular
expressions. (Sadahiro Tomoyuki)

=head2 $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)

=head1 Core Enhancements

=head2 state() variables

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

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

    use feature "state";

or by using the C<-E> command-line switch in one-liners.

See L<perlsub/"Persistent variables via state()">.

=head2 UNIVERSAL::DOES()

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

See L<< UNIVERSAL/"$obj->DOES( ROLE )" >>.

=head2 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.
(Nicholas Clark, Dave Mitchell)

=head2 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 L<perlfunc/require> for details. (Nicholas Clark)

=head2 MAD

MAD, which stands for I<Misc 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 C<-Dmad> to Configure. The
obtained perl isn't binary compatible with a regular perl 5.9.4, and has
space and speed penalties; moreover not all regression tests still pass
with it. (Larry Wall, Nicholas Clark)

=head1 Modules and Pragmas

=over 4

=item *

C<encoding::warnings> is now a lexical pragma. (Although on older perls,
which don't have support for lexical pragmas, it keeps its global
behaviour.) (Audrey Tang)

=item *

C<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 C<< 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)

=back

=head2 New Core Modules

=over 4

=item *

C<Hash::Util::FieldHash>, by Anno Siegel, has been added. This module
provides support for I<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.

=item *

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

=item *

C<Module::Load>, by Jos Boumans, has been added. It provides a single
interface to load Perl modules and F<.pl> files.

=item *

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

=item *

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

=item *

C<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.

=back

=head1 Utility Changes

=head2 config_data

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

=head1 Documentation

=head2 New manpage, perlpragma

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

=head2 New manpage, perlreguts

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

=head2 New manpage, perlunitut

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

=head1 Performance Enhancements

=head2 Memory optimisations

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

=head2 UTF-8 cache optimisation

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

=head2 Regular expressions

=over 4

=item 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)

=item 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)

=item 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. (Yves
Orton)

B<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 by the time 5.10 is
released.

=item 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)

=back

=head2 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)

=head1 Installation and Configuration Improvements

=head2 Relocatable installations

There is now Configure support for creating a relocatable perl tree. If
you Configure with C<-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 C<".../"> 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
C<-Duserelocatableinc> is that everything is relocated. The initial
install is done to the original configured prefix.

=head2 Ports

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

Perl has been reported to work on DragonFlyBSD.

=head2 Compilation improvements

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

=head2 New probes

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)

=head2 Windows build improvements

=over 4

=item Building XS extensions

Support for building XS extension modules with the free MinGW compiler has
been improved in the case where perl itself was built with the Microsoft
VC++ compiler. (ActiveState)

=item Support for 64-bit compiler

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

=back

=head1 Selected Bug Fixes

=head2 PERL5SHELL and tainting

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

=head2 Using *FILE{IO}

C<stat()> and C<-X> filetests now treat *FILE{IO} filehandles like *FILE
filehandles. (Steve Peters)

=head2 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)

=head2 Overloading and UTF-8

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

=head2 eval memory leaks fixed

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

=head2 Random device on Windows

In previous versions, perl would read the file F</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)

=head1 New or Changed Diagnostics

=over 4

=item State variable %s will be reinitialized

One can assign initial values to state variables, but not when they're
declared as a sub-part of a list assignment. See L<perldiag>.

=back

=head1 Changed Internals

A new file, F<mathoms.c>, contains functions that aren't used anymore in
the perl core, but remain around because modules out there might
still use them. They come from a factorization effort: for example, many
PP functions are now shared for several ops.

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

=head1 Known Problems

One warning test (number 263 in F<lib/warnings.t>) fails under UTF-8
locales.

Bytecode tests fail under several platforms. We are considering removing
support for byteloader and compiler before the 5.10.0 release.

=head1 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 B<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 C<perl -V>, will be sent off to perlbug@perl.org to be
analysed by the Perl porting team.

=head1 SEE ALSO

The F<Changes> file for exhaustive details on what changed.

The F<INSTALL> file for how to build Perl.

The F<README> file for general stuff.

The F<Artistic> and F<Copying> files for copyright information.

=cut