More perldelta entries from 5.9.3

[p5sagit/p5-mst-13.2.git] / pod / perl5100delta.pod

=head1 NAME

perldelta - what is new for perl 5.10.0

=head1 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.

=head1 Incompatible Changes

=head2 Packing and UTF-8 strings

=for XXX update this

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 C<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 C<use bytes>.

To be consistent with pack(), the C<C0> in unpack() templates indicates
that the data is to be processed in character mode, i.e. character by
character; on the contrary, C<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.

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

C<C0> and C<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, C<W>, which is intended to
replace the old C<C>. C<C> is kept for unsigned chars coded as bytes in
the strings internal representation. C<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<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<C>.

For consistency, C<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.

=head2 Byte/character count feature in unpack()

A new unpack() template character, C<".">, returns the number of bytes or
characters (depending on the selected encoding mode, see above) read so far.

=head2 The C<$*> and C<$#> variables have been removed

C<$*>, which was deprecated in favor of the C</s> and C</m> regexp
modifiers, has been removed.

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

Two new warnings, C<$#/$* is no longer supported>, have been added.

=head2 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.

=head2 Parsing of C<-f _>

The identifier C<_> is now forced to be a bareword after a filetest
operator. This solves a number of misparsing issues when a global C<_>
subroutine is defined.

=head2 C<:unique>

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

=head2 Scoping of the C<sort> pragma

The C<sort> pragma is now lexically scoped. Its effect used to be global.

=head2 Scoping of C<bignum>, C<bigint>, C<bigrat>

The three numeric pragmas C<bignum>, C<bigint> and C<bigrat> are now
lexically scoped. (Tels)

=head2 Effect of pragmas in eval

The compile-time value of the C<%^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("").

=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.

=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)

=head2 Tainting and printf

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

=head2 undef and signal handlers

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

=head2 strictures and array/hash dereferencing in defined()

C<defined @$foo> and C<defined %$bar> are now subject to C<strict 'refs'>
(that is, C<$foo> and C<$bar> shall be proper references there.)
(Nicholas Clark)

(However, C<defined(@foo)> and C<defined(%bar)> are discouraged constructs
anyway.)

=head2 C<(?p{})> has been removed

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

=head2 Pseudo-hashes have been removed

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

=head2 Removal of the bytecode compiler and of perlcc

C<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).

=head2 Removal of the JPL

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

=head2 Recursive inheritance detected earlier

Perl will now immediately throw an exception if you modify any package's
C<@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
C<$foo-E<gt>isa($bar)> lookup.

=head1 Core Enhancements

=head2 The C<feature> pragma

The C<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 C<strict> or C<warnings>.

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

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

=head2 New B<-E> command-line switch

B<-E> is equivalent to B<-e>, but it implicitly enables all
optional features (like C<use feature ":5.10">).

=head2 Defined-or operator

A new operator C<//> (defined-or) has been implemented.
The following statement:

    $a // $b

is merely equivalent to

   defined $a ? $a : $b

and

   $c //= $d;

can now be used instead of

   $c = $d unless defined $c;

The C<//> operator has the same precedence and associativity as C<||>.
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 L<perlop> for
details.

=head2 Switch and Smart Match operator

Perl 5 now has a switch statement. It's available when C<use feature
'switch'> is in effect. This feature introduces three new keywords,
C<given>, C<when>, and C<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 C<when> conditions is given in L<perlsyn/"Switch statements">.

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

This feature was contributed by Robin Houston.

=head2 Regular expressions

=over 4

=item Recursive Patterns

It is now possible to write recursive patterns without using the C<(??{})>
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 C<(?PARNO)> syntax (C<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

Note, users experienced with PCRE will find that the Perl implementation
of this feature differs from the PCRE one in that it is possible to
backtrack into a recursed pattern, whereas in PCRE the recursion is
atomic or "possessive" in nature. (Yves Orton)

=item 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 C<< (?<NAME>....) >>.
It's possible to backreference to a named buffer with the C<< \k<NAME> >>
syntax. In code, the new magical hashes C<%+> and C<%-> can be used to
access the contents of the capture buffers.

Thus, to replace all doubled chars, one could write

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

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

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

The C<%-> 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.

C<%+> and C<%-> are implemented as tied hashes through the new module
C<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)

=item 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 C<?+>, C<*+>, C<++>, C<{min,max}+> are now legal
quantifiers. (Yves Orton)

=item 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 L<perlre> for their descriptions. (Yves Orton)

=item Relative backreferences

A new syntax C<\g{N}> or C<\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 L<perlre/"Capture buffers">. (Yves Orton)

=item C<\K> escape

The functionality of Jeff Pinyan's module Regexp::Keep has been added to
the core. You can now use in regular expressions the special escape C<\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)

=item Vertical and horizontal whitespace, and linebreak

Regular expressions now recognize the C<\v> and C<\h> escapes, that match
vertical and horizontal whitespace, respectively. C<\V> and C<\H>
logically match their complements.

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

=item Unicode Character Classes

Perl's regular expression engine now contains support for matching on the
intersection of two Unicode character classes. You can also now refer to
user-defined character classes from within other user defined character
classes.

=back

=head2 C<say()>

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

=head2 Lexical C<$_>

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

    my $_;

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

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

In a scope where C<$_> has been lexicalized, you can still have access to
the global version of C<$_> by using C<$::_>, or, more simply, by
overriding the lexical declaration with C<our $_>.

=head2 The C<_> prototype

A new prototype character has been added. C<_> is equivalent to C<$> (it
denotes a scalar), but defaults to C<$_> if the corresponding argument
isn't supplied. 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 C<_> for some built-ins in appropriate cases (for
example, C<prototype('CORE::rmdir')>). (Rafael Garcia-Suarez)

=head2 UNITCHECK blocks

C<UNITCHECK>, a new special code block has been introduced, in addition to
C<BEGIN>, C<CHECK>, C<INIT> and C<END>.

C<CHECK> and C<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, C<UNITCHECK> blocks are executed
just after the unit which defined them has been compiled. See L<perlmod>
for more information. (Alex Gough)

=head2 New Pragma, C<mro>

A new pragma, C<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 mutiple inheritance hierachy. The
default MRO hasn't changed (DFS, for Depth First Search). Another MRO is
available: the C3 algorithm. See L<mro> for more information.
(Brandon Black)

Note that, due to changes in the implentation of class hierarchy search,
code that used to undef the C<*ISA> glob will most probably break. Anyway,
undef'ing C<*ISA> had the side-effect of removing the magic on the @ISA
array and should not have been done in the first place.

=head2 readpipe() is now overridable

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

=head2 default argument for readline()

readline() now defaults to C<*ARGV> if no argument is provided. (Rafael
Garcia-Suarez)

=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, Nicholas Clark)

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 Stacked filetest operators

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

=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 C<CLONE_SKIP()>

Perl has now support for the C<CLONE_SKIP> special subroutine. Like
C<CLONE>, C<CLONE_SKIP> is called once per package; however, it is called
just before cloning starts, and in the context of the parent thread. If it
returns a true value, then no objects of that class will be cloned. See
L<perlmod> for details. (Contributed by Dave Mitchell.)

=head2 Formats

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

=head2 Byte-order modifiers for pack() and unpack()

There are two new byte-order modifiers, C<E<gt>> (big-endian) and C<E<lt>>
(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 L<perlfunc/pack> and L<perlpacktut> for details.

=head2 C<no VERSION>

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

=head2 C<chdir>, C<chmod> and C<chown> on filehandles

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

=head2 OS groups

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

=head2 Recursive sort subs

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

=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 New internal variables

=over 4

=item C<${^RE_DEBUG_FLAGS}>

This variable controls what debug flags are in effect for the regular
expression engine when running under C<use re "debug">. See L<re> for
details.

=item C<${^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 L<perlrun> for details. (Contributed by Gisle Aas.)

=back

=head2 Miscellaneous

C<unpack()> now defaults to unpacking the C<$_> variable.

C<mkdir()> without arguments now defaults to C<$_>.

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

The B<-C> option can no longer be used on the C<#!> line. It wasn't
working there anyway.

=head2 PERLIO_DEBUG

The C<PERLIO_DEBUG> environment variable has no longer any effect for
setuid scripts and for scripts run with B<-T>.

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

=head2 UCD 5.0.0

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

=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 Pragmata

=head2 New modules

=over 4

=item *

C<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.

=item *

C<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, C<corelist>.

=item *

C<Math::BigInt::FastCalc> is an XS-enabled, and thus faster, version of
C<Math::BigInt::Calc>.

=item *

C<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 C<Archive::Tar> (see below).

=item *

C<IO::Zlib> is an C<IO::>-style interface to C<Compress::Zlib>.

=item *

C<Archive::Tar> is a module to manipulate C<tar> archives.

=item *

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

=item *

C<ExtUtils::CBuilder> and C<ExtUtils::ParseXS> have been added.

=back

=head1 Utility Changes

=over 4

=item 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
C<i> command.

Perl has a new -dt command-line flag, which enables threads support in the
debugger.

=item ptar

C<ptar> is a pure perl implementation of C<tar>, that comes with
C<Archive::Tar>.

=item ptardiff

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

=item shasum

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

=item corelist

The C<corelist> utility is now installed with perl (see L</"New modules">
above).

=item h2ph and h2xs

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

C<h2xs> implements a new option C<--use-xsloader> to force use of
C<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.

=item perlivp

C<perlivp> no longer checks for F<*.ph> files by default.  Use the new C<-a>
option to run I<all> tests.

=item find2perl

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

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

=back

=head1 New Documentation

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

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

=head1 Performance Enhancements

=over 4

=item In-place sorting

Sorting arrays in place (C<@a = sort @a>) is now optimized to avoid
making a temporary copy of the array.

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

=item 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.)

=item Trie optimization

The regexp engine now implements the trie optimization : it's able to
factorize common prefixes and suffixes in regular expressions. A new
special variable, ${^RE_TRIE_MAXBUF}, has been added to fine-tune this
optimization.

=item XS-assisted SWASHGET

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

=item 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 C<use POSIX;> now takes about
200K less memory.

=item C<PERL_DONT_CREATE_GVSV>

The new compilation flag C<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 L<perl588delta>
for details.

=item Weak references are cheaper

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

=item sort() enhancements

Salvador Fandiño provided improvements to reduce the memory usage of C<sort>
and to speed up some cases.

=back

=head1 Installation and Configuration Improvements

=head2 Compilation improvements

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

There is alpha support for relocatable @INC entries.

Parallel makes should work properly now, although there may still be problems
if C<make test> is instructed to run in parallel.

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.

Perl extensions on Windows now can be statically built into the Perl DLL,
thanks to a work by Vadim Konovalov.

=head2 New Or Improved Platforms

Perl is being ported to Symbian OS. See L<perlsymbian> for more
information.

The VMS port has been improved. See L<perlvms>.

DynaLoader::dl_unload_file() now works on Windows.

Portability of Perl on various recent compilers on Windows has been
improved (Borland C++, Visual C++ 7.0).

=head2 Module auxiliary files

README files and changelogs for CPAN modules bundled with perl are no
longer installed.

=head1 Selected Bug Fixes

=over 4

=item strictures in regexp-eval blocks

C<strict> wasn't in effect in regexp-eval blocks (C</(?{...})/>).

=item C<defined $$x>

C<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 C<Can't use string as a
SCALAR ref while "strict refs" in use>.

=item Calling CORE::require()

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

=item 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 C<< -> >> was required.

=item C<no warnings 'category'> works correctly with -w

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

=item threads and memory usage

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

=item chr() and negative values

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

=back

=head1 New or Changed Diagnostics

=over 4

=item Deprecated use of my() in false conditional

A new deprecation warning, I<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 L<perldiag>. Use C<state> variables instead.

=item !=~ should be !~

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

=item Newline in left-justified string

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

=item Too late for "-T" option

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

=item "%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 C<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 C<"our" variable %s redeclared> warning.

=item 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.

=item perl -V

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

=back

=head1 Changed Internals

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

=head2 Reordering of SVt_* constants

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

=head2 Removal of CPP symbols

The C preprocessor symbols C<PERL_PM_APIVERSION> and
C<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.

=head2 Less space is used by ops

The C<BASEOP> structure now uses less space. The C<op_seq> field has been
removed and replaced by the one-bit fields C<op_opt>. C<op_type> is now 9
bits long. (Consequently, the C<B::OP> class doesn't provide an C<seq>
method anymore.)

=head2 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 C<-DT>.

=head2 Use of C<const>

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

=head2 Mathoms

A new file, F<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 C<-DNO_MATHOMS> in the compiler flags.

=head2 C<AvFLAGS> has been removed

The C<AvFLAGS> macro has been removed.

=head2 C<av_*> changes

The C<av_*()> functions, used to manipulate arrays, no longer accept null
C<AV*> parameters.

=head2 B:: modules inheritance changed

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

=head1 New Tests

=head1 Known Problems

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

=head1 Platform Specific Problems

=head1 Reporting Bugs

=head1 SEE ALSO

The F<Changes> file and the perl590delta to perl595delta man pages 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