Add the author list for 5.12.0

[perl5.git] / pod / perl5120delta.pod

=head1 NAME

perl5120delta - what is new for perl v5.12.0

FIX ME BEFORE RELEASE

UPDATED MODULE LIST NEEDS TO BE GENERATED

=head1 DESCRIPTION

This document describes differences between the 5.10.0 release and
the 5.12.0 release.

Many of the bug fixes in 5.12.0 are already included in the 5.10.1
maintenance release. 

You can see the list of those changes in the 5.10.1 release notes (L<perl5101delta>).


=head1 Core Enhancements

=head2 New C<package NAME VERSION> syntax

This new syntax allows a module author to set the $VERSION of a namespace
when the namespace is declared with 'package'. It eliminates the need
for C<our $VERSION = ...> and similar constructs. E.g.

      package Foo::Bar 1.23;
      # $Foo::Bar::VERSION == 1.23

There are several advantages to this:

=over 

=item *

C<$VERSION> is parsed in exactly the same way as C<use NAME VERSION>

=item *

C<$VERSION> is set at compile time

=item *

C<$VERSION> is a version object that provides proper overloading of
comparision operators so comparing C<$VERSION> to decimal (1.23) or
dotted-decimal (v1.2.3) version numbers works correctly.

=item *

Eliminates C<$VERSION = ...> and C<eval $VERSION> clutter

=item *

As it requires VERSION to be a numeric literal or v-string
literal, it can be statically parsed by toolchain modules
without C<eval> the way MM-E<gt>parse_version does for C<$VERSION = ...>

=item *

It does not break old code with only C<package NAME>, but code that uses
C<package NAME VERSION> will need to be restricted to perl 5.12.0 or newer
This is analogous to the change to C<open> from two-args to three-args.
Users requiring the latest Perl will benefit, and perhaps after several
years, it will become a standard practice.

=back

However, C<package NAME VERSION> requires a new, 'strict' version
number format. See L<"Version number formats"> for details.


=head2 The C<...> operator

A new operator, C<...>, nicknamed the Yada Yada operator, has been added.
It is intended to mark placeholder code that is not yet implemented.
See L<perlop/"Yada Yada Operator">. (chromatic)

=head2 Implicit strictures

Using the C<use VERSION> syntax with a version number greater or equal
to 5.11.0 will lexically enable strictures just like C<use strict>
would do (in addition to enabling features.) The following:

    use 5.12.0;

means:

    use strict;
    use feature ':5.12';

=head2 Unicode improvements

Perl 5.12 comes with Unicode 5.2, the latest version available to
us at the time of release.  This version of Unicode was released in
October 2009. See L<http://www.unicode.org/versions/Unicode5.2.0> for
further details about what's changed in this version of the standard.
See L<perlunicode> for instructions on installing and using other versions
of Unicode.

Additionally, Perl's developers have significantly improved Perl's Unicode
implementation. For full details, see L</Unicode overhaul> below.

=head2 Y2038 compliance

Perl's core time-related functions are now Y2038 compliant. (It may not mean much to you, but your kids will love it!)

=head2 qr overloading

It is now possible to overload the C<qr//> operator, that is,
conversion to regexp, like it was already possible to overload
conversion to boolean, string or number of objects. It is invoked when
an object appears on the right hand side of the C<=~> operator or when
it is interpolated into a regexp. See L<overload>.

=head2 Pluggable keywords

Extension modules can now cleanly hook into the Perl parser to define
new kinds of keyword-headed expression and compound statement. The
syntax following the keyword is defined entirely by the extension. This
allow a completely non-Perl sublanguage to be parsed inline, with the
correct ops cleanly generated. 

See L<perlapi/PL_keyword_plugin> for the mechanism. The Perl core
source distribution also includes a new module
L<XS::APItest::KeywordRPN>, which implements reverse Polish notation
arithmetic via pluggable keywords. This module is mainly used for test
purposes, and is not normally installed, but also serves as an example
of how to use the new mechanism.

Perl's developers consider this feature to be experimental. We may remove
it or change it in a backwards-incompatible way in Perl 5.14.

=head2 APIs for more internals

The lowest layers of the lexer and parts of the pad system now have C
APIs available to XS extensions. These are necessary to support proper
use of pluggable keywords, but have other uses too. The new APIs are
experimental, and only cover a small proportion of what would be
necessary to take full advantage of the core's facilities in these
areas. It is intended that the Perl 5.13 development cycle will see the
addition of a full range of clean, supported interfaces.

Perl's developers consider this feature to be experimental. We may remove
it or change it in a backwards-incompatible way in Perl 5.14.

=head2 Overridable function lookup

Where an extension module hooks the creation of rv2cv ops to modify the
subroutine lookup process, this now works correctly for bareword
subroutine calls. This means that prototypes on subroutines referenced
this way will be processed correctly. (Previously bareword subroutine
names were initially looked up, for parsing purposes, by an unhookable
mechanism, so extensions could only properly influence subroutine names
that appeared with an C<&> sigil.)

=head2 A proper interface for pluggable Method Resolution Orders

As of Perl 5.12.0 there is a new interface for plugging and using method
resolution orders other than the default linear depth first search.
The C3 method resolution order added in 5.10.0 has been re-implemented as
a plugin, without changing its Perl-space interface. See L<perlmroapi> for
more information.



=head2 C<\N> experimental regex escape

Perl now supports C<\N>, a new regex escape which you can think of as
the inverse of C<\n>. It will match any character that is not a newline,
independently from the presence or absence of the single line match
modifier C</s>. It is not usable within a character class.  C<\N{3}>
means to match 3 non-newlines; C<\N{5,}> means to match at least 5.
C<\N{NAME}> still means the character or sequence named C<NAME>, but
C<NAME> no longer can be things like C<3>, or C<5,>.

This will break a L<custom charnames translator|charnames/CUSTOM
TRANSLATORS> which allows numbers for character names, as C<\N{3}> will
now mean to match 3 non-newline characters, and not the character whose
name is C<3>. (No name defined by the Unicode standard is a number,
so only custom translators might be affected.)

Perl's developers are somewhat concerned about possible user confusion
with the existing C<\N{...}> construct which matches characters by their
Unicode name. Consequently, this feature is experimental. We may remove
it or change it in a backwards-incompatible way in Perl 5.14.

=head2 DTrace support

Perl now has some support for DTrace. See "DTrace support" in F<INSTALL>.

=head2 Support for C<configure_requires> in CPAN module metadata

Both C<CPAN> and C<CPANPLUS> now support the C<configure_requires> keyword
in the F<META.yml> metadata file included in most recent CPAN distributions.
This allows distribution authors to specify configuration prerequisites that
must be installed before running F<Makefile.PL> or F<Build.PL>.

See the documentation for C<ExtUtils::MakeMaker> or C<Module::Build> for more
on how to specify C<configure_requires> when creating a distribution for CPAN.

=head2 C<each> is now more flexible

The C<each> function can now operate on arrays.

=head2 C<$,> flexibility

The variable C<$,> may now be tied.

=head2 // in where clauses

// now behaves like || in when clauses

=head2 Enabling warnings from your shell environment

You can now set C<-W> from the C<PERL5OPT> environment variable

=head2 C<delete local>

C<delete local> now allows you to locally delete a hash entry.

=head2 New support for Abstract namespace sockets

Abstract namespace sockets are Linux-specific socket type that live in
AF_UNIX family, slightly abusing it to be able to use arbitrary
character arrays as addresses: They start with nul byte and are not
terminated by nul byte, but with the length passed to the socket()
system call.

=head2 32-bit limit on substr arguments removed

The 32-bit limit on C<substr> arguments has now been removed. The full range
of the system's signed and unsigned integers is now available for the C<pos>
and C<len> arguments.

=head1 Potentially Incompatible Changes

=head2 Deprecations warn by default

Perl now defaults to issuing a warning if a deprecated language feature
is used.

To disable this feature in a given lexical scope, you should use C<no
warnings 'deprecated';> For information about which language features
are deprecated and explanations of various deprecation warnings, please
see L<perldiag.pod>. See L</Deprecations> below for the list of features
and modules Perl's developers have deprecated as part of this release.

=head2 Version number formats

Acceptable version number formats have been formalized into "strict" and
"lax" rules. C<package NAME VERSION> takes a strict version number.
C<UNIVERSAL::VERSION> and the L<version> object constructors take lax
version numbers. Providing an invalid version will result in a fatal
error. The version argument in C<use NAME VERSION> is first parsed as a
numeric literal or v-string and then passed to C<UNIVERSAL::VERSION>
(and must then pass the "lax" format test).

These formats are documented fully in the L<version> module. To a first
approximation, a "strict" version number is a positive decimal number
(integer or decimal-fraction) without exponentiation or else a
dotted-decimal v-string with a leading 'v' character and at least three
components. A "lax" version number allows v-strings with fewer than
three components or without a leading 'v'. Under "lax" rules, both
decimal and dotted-decimal versions may have a trailing "alpha"
component separated by an underscore character after a fractional or
dotted-decimal component.

The L<version> module adds C<version::is_strict> and C<version::is_lax>
functions to check a scalar against these rules.

=head2 @INC reorganization

In C<@INC>, C<ARCHLIB> and C<PRIVLIB> now occur after after the current
version's C<site_perl> and C<vendor_perl>.  Modules installed into
C<site_perl> and C<vendor_perl> will now be loaded in preference to
those installed in C<ARCHLIB> and C<PRIVLIB>.

=head2 Switch statement changes

The C<given>/C<when> switch statement handles complex statements better
than Perl 5.10.0 did (These enhancements are also available in
5.10.1 and subsequent 5.10 releases.) There are two new cases where
C<when> now interprets its argument as a boolean, instead of an
expression to be used in a smart match:

=over

=item flip-flop operators

The C<..> and C<...> flip-flop operators are now evaluated in boolean
context, following their usual semantics; see L<perlop/"Range Operators">.

Note that, as in perl 5.10.0, C<when (1..10)> will not work to test
whether a given value is an integer between 1 and 10; you should use
C<when ([1..10])> instead (note the array reference).

However, contrary to 5.10.0, evaluating the flip-flop operators in boolean
context ensures it can now be useful in a C<when()>, notably for
implementing bistable conditions, like in:

    when (/^=begin/ .. /^=end/) {
      # do something
    }

=item defined-or operator

A compound expression involving the defined-or operator, as in
C<when (expr1 // expr2)>, will be treated as boolean if the first
expression is boolean. (This just extends the existing rule that applies
to the regular or operator, as in C<when (expr1 || expr2)>.)

=back

=head2 Smart match changes

Since Perl 5.10.0, Perl's developers have made a number of changes to
the smart match operator. These, of course, also alter the behaviour
of the switch statements where smart matching is implicitly used.
These changes were also made for the 5.10.1 release, and will remain in
subsequent 5.10 releases.

=head3 Changes to type-based dispatch

The smart match operator C<~~> is no longer commutative. The behaviour of
a smart match now depends primarily on the type of its right hand
argument. Moreover, its semantics have been adjusted for greater
consistency or usefulness in several cases. While the general backwards
compatibility is maintained, several changes must be noted:

=over 4

=item *

Code references with an empty prototype are no longer treated specially.
They are passed an argument like the other code references (even if they
choose to ignore it).

=item *

C<%hash ~~ sub {}> and C<@array ~~ sub {}> now test that the subroutine
returns a true value for each key of the hash (or element of the
array), instead of passing the whole hash or array as a reference to
the subroutine.

=item *

Due to the commutativity breakage, code references are no longer
treated specially when appearing on the left of the C<~~> operator,
but like any vulgar scalar.

=item *

C<undef ~~ %hash> is always false (since C<undef> can't be a key in a
hash). No implicit conversion to C<""> is done (as was the case in perl
5.10.0).

=item *

C<$scalar ~~ @array> now always distributes the smart match across the
elements of the array. It's true if one element in @array verifies
C<$scalar ~~ $element>. This is a generalization of the old behaviour
that tested whether the array contained the scalar.

=back

The full dispatch table for the smart match operator is given in
L<perlsyn/"Smart matching in detail">.

=head3 Smart match and overloading

According to the rule of dispatch based on the rightmost argument type,
when an object overloading C<~~> appears on the right side of the
operator, the overload routine will always be called (with a 3rd argument
set to a true value, see L<overload>.) However, when the object will
appear on the left, the overload routine will be called only when the
rightmost argument is a simple scalar. This way, distributivity of smart
match across arrays is not broken, as well as the other behaviours with
complex types (coderefs, hashes, regexes). Thus, writers of overloading
routines for smart match mostly need to worry only with comparing
against a scalar, and possibly with stringification overloading; the
other common cases will be automatically handled consistently.

C<~~> will now refuse to work on objects that do not overload it (in order
to avoid relying on the object's underlying structure). (However, if the
object overloads the stringification or the numification operators, and
if overload fallback is active, it will be used instead, as usual.)

=head2 Other potentially incompatible changes

=over 4

=item *

The definitions of a number of Unicode properties have changed to match
those of the current Unicode standard. These are listed above under
L</Unicode overhaul>. This change may break code that expects the old definitions.

=item *

The boolkeys op has moved to the group of hash ops. This breaks binary
compatibility.

=item *

Filehandles are now always blessed into C<IO::File>.

The previous behaviour was to bless Filehandles into L<FileHandle>
(an empty proxy class) if it was loaded into memory and otherwise
to bless them into C<IO::Handle>.

=item *

The semantics of C<use feature :5.10*> have changed slightly.
See L<"Modules and Pragmata"> for more information.

=item *

Perl's developers now use git, rather than Perforce.  This should be
a purely internal change only relevant to people actively working on
the core.  However, you may see minor difference in perl as a consequence
of the change.  For example in some of details of the output of C<perl
-V>. See L<perlrepository> for more information.

=item *

As part of the C<Test::Harness> 2.x to 3.x upgrade, the experimental
C<Test::Harness::Straps> module has been removed.
See L</"Modules and Pragmata"> for more details.

=item *

As part of the C<ExtUtils::MakeMaker> upgrade, the
C<ExtUtils::MakeMaker::bytes> and C<ExtUtils::MakeMaker::vmsish> modules
have been removed from this distribution.

=item *

C<Module::CoreList> no longer contains the C<%:patchlevel> hash.


=item *

C<length undef> now returns undef.

=item *

Unsupported private C API functions are now declared "static" to prevent
leakage to Perl's public API.

=item *

To support the bootstrapping process, F<miniperl> no longer builds with
UTF-8 support in the regexp engine.

This allows a build to complete with PERL_UNICODE set and a UTF-8 locale.
Without this there's a bootstrapping problem, as miniperl can't load the UTF-8
components of the regexp engine, because they're not yet built.

=item *

F<miniperl>'s @INC is now restricted to just C<-I...>, the split of
C<$ENV{PERL5LIB}>, and "C<.>"

=item *

A space or a newline is now required after a C<"#line XXX"> directive.

=item *

Tied filehandles now have an additional method EOF which provides the EOF type

=item *

To better match all other flow control statements, C<foreach> may no
longer be used as an attribute.

=back


=head1 Deprecations

From time to time, Perl's developers find it necessary to deprecate
features or modules we've previously shipped as part of the core
distribution. We are well aware of the pain and frustration that a
backwards-incompatible change to Perl can cause for developers building
or maintaining software in Perl. You can be sure that when we deprecate
a functionality or syntax, it isn't a choice we make lightly. Sometimes,
we choose to deprecate functionality or syntax because it was found to
be poorly designed or implemented. Sometimes, this is because they're
holding back other features or causing performance problems. Sometimes,
the reasons are more complex. Wherever possible, we try to keep deprecated
functionality available to developers in its previous form for at least
one major release. So long as a deprecated feature isn't actively
disrupting our ability to maintain and extend Perl, we'll try to leave
it in place as long as possible.

The following items are now deprecated:

=over

=item suidperl

C<suidperl> is no longer part of Perl. It used to provide a mechanism to
emulate setuid permission bits on systems that don't support it properly.


=item Use of C<:=> to mean an empty attribute list

An accident of Perl's parser meant that these constructions were all
equivalent:

    my $pi := 4;
    my $pi : = 4;
    my $pi :  = 4;

with the C<:> being treated as the start of an attribute list, which
ends before the C<=>. As whitespace is not significant here, all are
parsed as an empty attribute list, hence all the above are equivalent
to, and better written as

    my $pi = 4;

because no attribute processing is done for an empty list.

As is, this meant that C<:=> cannot be used as a new token, without
silently changing the meaning of existing code. Hence that particular
form is now deprecated, and will become a syntax error. If it is
absolutely necessary to have empty attribute lists (for example,
because of a code generator) then avoid the warning by adding a space
before the C<=>.

=item C<< UNIVERSAL->import() >>

The method C<< UNIVERSAL->import() >> is now deprecated. Attempting to
pass import arguments to a C<use UNIVERSAL> statement will result in a
deprecation warning.


=item Use of "goto" to jump into a construct

Using C<goto> to jump from an outer scope into an inner scope is now
deprecated. This rare use case was causing problems in the
implementation of scopes.

=item Custom character names in \N{name} that don't look like names

In C<\N{I<name>}>, I<name> can be just about anything. The standard Unicode
names have a very limited domain, but a custom name translator could create
names that are, for example, made up entirely of punctuation symbols. It is
now deprecated to make names that don't begin with an alphabetic character, and
aren't alphanumeric or contain other than a very few other characters,
namely spaces, dashes, parentheses and colons. Because of the added meaning of
C<\N> (See L</C<\N> experimental regex escape>), names that look like curly
brace -enclosed quantifiers won't work. For example, C<\N{3,4}> now means to
match 3 to 4 non-newlines; before a custom name C<3,4> could have been created.

=item Deprecated Modules

The following modules will be removed from the core distribution in a future
release, and should be installed from CPAN instead. Distributions on CPAN
which require these should add them to their prerequisites. The core versions
of these modules warnings will issue a deprecation warning.

If you ship a packaged version of Perl, either alone or as part of a larger
system, then you should carefully consider the reprecussions of core module
deprecations. You may want to consider shipping your default build of
Perl with packages for some or all deprecated modules which install into
C<vendor> or C<site> perl library directories. This will inhibit the 
deprecation warnings.

Alternatively, you may want to consider patching F<lib/deprecate.pm>
to provide deprecation warnings specific to your packaging system or
distribution of Perl, consistent with how your packaging system or
distribution manages a staged transition from a release where the
installation of a single package provides the given functionality, to a later
release where the system administrator needs to know to install multiple
packages to get that same functionality.

=over

=item L<Class::ISA>

=item L<Pod::Plainer>

=item L<Shell>

=item L<Switch>

Switch is buggy and should be avoided. You may find Perl's new
C<given>/C<when> feature a suitable replacement.  See L<perlsyn/"Switch
statements"> for more information.

=back

=item Assignment to $[

=item Use of the attribute :locked on subroutines

=item Use of "locked" with the attributes pragma

=item Use of "unique" with the attributes pragma

=item Perl_pmflag

C<Perl_pmflag> is no longer part of Perl's public API. Calling it now
generates a deprecation warning, and it will be removed in a future
release. Although listed as part of the API, it was never documented,
and only ever used in F<toke.c>, and prior to 5.10, F<regcomp.c>. In
core, it has been replaced by a static function.

=item Numerous Perl 4-era libraries

F<termcap.pl>, F<tainted.pl>, F<stat.pl>, F<shellwords.pl>, F<pwd.pl>,
F<open3.pl>, F<open2.pl>, F<newgetopt.pl>, F<look.pl>, F<find.pl>,
F<finddepth.pl>, F<importenv.pl>, F<hostname.pl>, F<getopts.pl>,
F<getopt.pl>, F<getcwd.pl>, F<flush.pl>, F<fastcwd.pl>, F<exceptions.pl>,
F<ctime.pl>, F<complete.pl>, F<cacheout.pl>, F<bigrat.pl>, F<bigint.pl>,
F<bigfloat.pl>, F<assert.pl>, F<abbrev.pl>, F<dotsh.pl>, and
F<timelocal.pl> are all now deprecated. Using them will incur a warning.


=back

=head1 Unicode overhaul

Perl's developers have made a concerted effort to update Perl to be in
sync with the latest Unicode standard. Changes for this include:

Perl can now handle every Unicode character property. New documentation,
L<perluniprops>, lists all available non-Unihan character properties. By
default, perl does not expose Unihan, deprecated or Unicode-internal
properties.  See below for more details on these; there is also a section
in the pod listing them, and explaining why they are not exposed.

Perl now fully supports the Unicode compound-style of using C<=> and C<:>
in writing regular expressions: C<\p{property=value}> and
C<\p{property:value}> (both of which mean the same thing).

Perl now fully supports the Unicode loose matching rules for text
between the braces in C<\p{...}> constructs. In addition, Perl allows
underscores between digits of numbers.

Perl now accepts all the Unicode-defined synonyms for properties and property values.

C<qr/\X/>, which matches a Unicode logical character, has been expanded to work
better with various Asian languages. It now is defined as an I<extended
grapheme cluster>. (See L<http://www.unicode.org/reports/tr29/>).
Anything matched previously and that made sense will continue to be
accepted.   Additionally:

=over

=item *

C<\X> will not break apart a C<S<CR LF>> sequence.

=item *

C<\X> will now match a sequence which includes the C<ZWJ> and C<ZWNJ> characters.

=item *

C<\X> will now always match at least one character, including an initial mark.
Marks generally come after a base character, but it is possible in Unicode to
have them in isolation, and C<\X> will now handle that case, for example at the
beginning of a line, or after a C<ZWSP>. And this is the part where C<\X>
doesn't match the things that it used to that don't make sense. Formerly, for
example, you could have the nonsensical case of an accented LF.

=item *

C<\X> will now match a (Korean) Hangul syllable sequence, and the Thai and Lao
exception cases.

=back

Otherwise, this change should be transparent for the non-affected languages.

C<\p{...}> matches using the Canonical_Combining_Class property were
completely broken in previous releases of Perl.  They should now work correctly.

Before Perl 5.12, the Unicode C<Decomposition_Type=Compat> property and a
Perl extension had the same name, which led to neither matching all the
correct values (with more than 100 mistakes in one, and several thousand
in the other). The Perl extension has now been renamed to be
C<Decomposition_Type=Noncanonical> (short: C<dt=noncanon>). It has the same
meaning as was previously intended, namely the union of all the
non-canonical Decomposition types, with Unicode C<Compat> being just one of
those.

C<\p{Decomposition_Type=Canonical}> now includes the Hangul syllables.

C<\p{Uppercase}> and C<\p{Lowercase}> now work as the Unicode standard says they should.
This means they each match a few more characters than they used to.

C<\p{Cntrl}> now matches the same characters as C<\p{Control}>. This means it
no longer will match Private Use (gc=co), Surrogates (gc=cs), nor Format
(gc=cf) code points. The Format code points represent the biggest
possible problem. All but 36 of them are either officially deprecated
or strongly discouraged from being used. Of those 36, likely the most
widely used are the soft hyphen (U+00AD), and BOM, ZWSP, ZWNJ, WJ, and
similar characters, plus bidirectional controls.

C<\p{Alpha}> now matches the same characters as C<\p{Alphabetic}>. Before 5.12, Perl's definition
definition included a number of things that aren't really alpha (all
marks) while omitting many that were. The
definitions of C<\p{Alnum}> and C<\p{Word}> depend on Alpha's definition and have changed accordingly.

C<\p{Word}> no longer incorrectly matches non-word characters such as fractions.

C<\p{Print}> no longer matches the line control characters: Tab, LF, CR,
FF, VT, and NEL. This brings it in line with standards and the documentation.

C<\p{XDigit}> now matches the same characters as C<\p{Hex_Digit}>. This
means that in addition to the characters it currently matches,
C<[A-Fa-f0-9]>, it will also match the 22 fullwidth equivalents, for
example U+FF10: FULLWIDTH DIGIT ZERO.

The Numeric type property has been extended to include the Unihan
characters.

There is a new Perl extension, the 'Present_In', or simply 'In',
property. This is an extension of the Unicode Age property, but
C<\p{In=5.0}> matches any code point whose usage has been determined
I<as of> Unicode version 5.0. The C<\p{Age=5.0}> only matches code points
added in I<precisely> version 5.0.

A number of properties now have the correct values for unassigned
code points. The affected properties are
Bidi_Class, East_Asian_Width, Joining_Type, Decomposition_Type,
Hangul_Syllable_Type, Numeric_Type, and Line_Break.

The Default_Ignorable_Code_Point, ID_Continue, and ID_Start properties
are now up to date with current Unicode definitions.

Earlier versions of Perl erroneously exposed certain properties that are supposed to be Unicode internal-only.
Use of these in regular expressions will now generate, if enabled, a deprecation warning message.
The properties are: Other_Alphabetic, Other_Default_Ignorable_Code_Point,
Other_Grapheme_Extend, Other_ID_Continue, Other_ID_Start, Other_Lowercase,
Other_Math, and Other_Uppercase.

It is now possible to change which Unicode properties Perl understands
on a per-installation basis. As mentioned above, certain properties
are turned off by default.  These include all the Unihan properties
(which should be accessible via the CPAN module Unicode::Unihan) and any
deprecated or Unicode internal-only property that Perl has never exposed.

The generated files in the C<lib/unicore/To> directory are now more
clearly marked as being stable, directly usable by applications.
New hash entries in them give the format of the normal entries,
which allows for easier machine parsing. Perl can generate files
in this directory for any property, though most are suppressed.
You can find instructions for changing which are written in L<perluniprops>.



=head1 Modules and Pragmata

=head2 Notable new Modules and Pragmata

=over 4

=item *

C<autodie> is a new lexically-scoped alternative for the C<Fatal> module.
The bundled version is 2.06_01. Note that in this release, using a string
eval when C<autodie> is in effect can cause the autodie behaviour to leak
into the surrounding scope. See L<autodie/"BUGS"> for more details.

=item *

C<Compress::Raw::Bzip2>

=item *

C<overloading> allows you to lexically disable or enable overloading
for some or all operations. (Yuval Kogman)

=item *

C<parent> establishes an ISA relationship with base classes at compile
time. It provides the key feature of C<base> without further unwanted
behaviors.

=item *

C<Parse::CPAN::Meta>

=back

=head2 Notable changes to Modules and Pragmata

=over 4

=item *

C<charnames> now contains the Unicode F<NameAliases.txt> database file.
This has the effect of adding some extra C<\N> character names that
formerly wouldn't have been recognised; for example, C<"\N{LATIN CAPITAL
LETTER GHA}">.

=item *

In C<feature>, the meaning of the C<:5.10> and C<:5.10.X> feature bundles has
changed slightly. The last component, if any (i.e. C<X>) is simply ignored.
This is predicated on the assumption that new features will not, in
general, be added to maintenance releases. So C<:5.10> and C<:5.10.X>
have identical effect. This is a change to the behaviour documented for
5.10.0.

C<feature> now includes the C<unicode_strings> feature:

    use feature "unicode_strings";

This pragma turns on Unicode semantics for the case-changing operations
(C<uc>, C<lc>, C<ucfirst>, C<lcfirst>) on strings that don't have the
internal UTF-8 flag set, but that contain single-byte characters between
128 and 255.

=item *

C<mro> is now implemented as an XS extension. The documented interface has not
changed. Code relying on the implementation detail that some C<mro::>
methods happened to be available at all times gets to "keep both pieces".

=item *

C<diagnostics> now supports %.0f formatting internally.

=item *

C<overload> now allow overloading of 'qr'.

=item *

C<diagnostics> no longer suppresses C<Use of uninitialized value in range
(or flip)> warnings. [perl #71204]

C<less> now includes the C<stash_name> method to allow subclasses of
C<less> to pick where in %^H to store their stash.

=item *

C<version> now has support for L</Version number formats> as described earlier
in this document and in its own documentation.

=item *

C<warnings> has a new C<warnings::fatal_enabled()> function.  It also includes a new C<illegalproto> warning category. See also L</New or Changed Diagnostics> for this change.

=back


=head2 Removed Modules and Pragmata

=over 4

=item *

C<attrs>

The C<attrs> pragma has been removed. It had been marked as deprecated
since 5.6.0.

=item *

C<Devel::DProf::V> is no longer part of the Perl core.

=back

=head2 Deprecated Modules and Pragmata

See L</Deprecated Modules> above.

=head1 Documentation

=head2 New Documentation

=over 4

=item *

L<perlhaiku> contains instructions on how to build perl for the Haiku platform.

=item *

L<perlmroapi> describes the new interface for pluggable Method Resolution Orders.

=item *

L<perlperf>, by Richard Foley, provides an introduction to the use of
performance and optimization techniques which can be used with particular
reference to perl programs.

=item *

L<perlrepository> describes how to access the perl source using the I<git> version
control system.

=item *

L<perlpolicy> extends the "Social contract about contributed modules" into
the beginnings of a document on Perl porting policies.

=back

=head2 Changes to Existing Documentation


=over


=item *

The various large F<Changes*> files (which listed every change made to perl
over the last 18 years) have been removed, and replaced by a small file,
also called F<Changes>, which just explains how that same information may
be extracted from the git version control system.

=item *

F<Porting/patching.pod> has been deleted, as it mainly described
interacting with the old Perforce-based repository, which is now obsolete.
Information still relevant has been moved to L<perlrepository>.


=item *

The syntax C<unless (EXPR) BLOCK else BLOCK> is now documented as valid, as
is the syntax C<unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK>,
although actually using the latter may not be the best idea for the
readability of your source code.


=item *

Documented -X overloading.

=item *

Documented that C<when()> treats specially most of the filetest operators

=item *

Documented C<when> as a syntax modifier.

=item *

Eliminated "Old Perl threads tutorial", which described 5005 threads.

F<pod/perlthrtut.pod> is the same material reworked for ithreads.

=item *

Correct previous documentation: v-strings are not deprecated

With version objects, we need them to use MODULE VERSION syntax. This
patch removes the deprecation notice.

=item *

Security contact information is now part of L<perlsec>.

=item *

A significant fraction of the core documentation has been updated to clarify
the behavior of Perl's Unicode handling.

Much of the remaining core documentation has been reviewed and edited
for clarity, consistent use of language, and to fix the spelling of Tom
Christiansen's name.

=item *

The Pod specification (L<perlpodspec>) has been updated to bring the
specification in line with modern usage already supported by most Pod
systems. A parameter string may now follow the format name in a
"begin/end" region. Links to URIs with a text description are now
allowed. The usage of C<LE<lt>"section"E<gt>> has been marked as
deprecated.

=item *

L<if.pm|if> has been documented in L<perlfunc/use> as a means to get
conditional loading of modules despite the implicit BEGIN block around
C<use>.

=item *

The documentation for C<$1> in perlvar.pod has been clarified.

=item *

C<\N{U+I<wide hex char>}> is now documented.

=back

=head1 Selected Performance Enhancements

=over 4

=item *

A new internal cache means that C<isa()> will often be faster.

=item *

The implementation of C<C3> Method Resolution Order has been optimised -
linearisation for classes with single inheritance is 40% faster. Performance
for multiple inheritance is unchanged.

=item *

Under C<use locale>, the locale-relevant information is now cached on
read-only values, such as the list returned by C<keys %hash>. This makes
operations such as C<sort keys %hash> in the scope of C<use locale> much
faster.

=item *

Empty C<DESTROY> methods are no longer called.

=item *

C<Perl_sv_utf8_upgrade()> is now faster.

=item *

C<keys> on empty hash is now faster.

=item *

C<if (%foo)> has been optimized to be faster than C<if (keys %foo)>.

=item *

Reversing an array to itself (as in C<@a = reverse @a>) in void context
now happens in-place and is several orders of magnitude faster than it
used to be. It will also preserve non-existent elements whenever
possible, i.e. for non magical arrays or tied arrays with C<EXISTS> and
C<DELETE> methods.

=back

=head1 Installation and Configuration Improvements

=over 4

=item *

L<perlapi>, L<perlintern>, L<perlmodlib> and L<perltoc> are now all
generated at build time, rather than being shipped as part of the release.

=item *

If C<vendorlib> and C<vendorarch> are the same, then they are only added to
C<@INC> once.

=item *

C<$Config{usedevel}> and the C-level C<PERL_USE_DEVEL> are now defined if
perl is built with  C<-Dusedevel>.

=item *

F<Configure> will enable use of C<-fstack-protector>, to provide protection
against stack-smashing attacks, if the compiler supports it.

=item *

F<Configure> will now determine the correct prototypes for re-entrant
functions and for C<gconvert> if you are using a C++ compiler rather
than a C compiler.

=item *

On Unix, if you build from a tree containing a git repository, the
configuration process will note the commit hash you have checked out, for
display in the output of C<perl -v> and C<perl -V>. Unpushed local commits
are automatically added to the list of local patches displayed by
C<perl -V>.

=item *

Perl now supports SystemTap's C<dtrace> compatibility layer and an
issue with linking C<miniperl> has been fixed in the process.

=item *

perldoc now uses C<less -R> instead of C<less> for improved behaviour
in the face of C<groff>'s new usage of ANSI escape codes.

=item *


C<perl -V> now reports use of the compile-time options C<USE_PERL_ATOF> and
C<USE_ATTRIBUTES_FOR_PERLIO>.

=item *

As part of the flattening of F<ext>, all extensions on all platforms are
built by F<make_ext.pl>. This replaces the Unix-specific
F<ext/util/make_ext>, VMS-specific F<make_ext.com> and Win32-specific
F<win32/buildext.pl>.

=back

=head1 Internal Changes

Each release of Perl sees numerous internal changes which shouldn't
affect day to day usage but may still be notable for developers working
with Perl's source code.

=over

=item *

The J.R.R. Tolkien quotes at the head of C source file have been checked and
proper citations added, thanks to a patch from Tom Christiansen.

=item *

The internal structure of the dual-life modules traditionally found in
the F<lib/> and F<ext/> directories y in the perl source has changed
significantly. Where possible, dual-lifed modules have been extracted
from F<lib/> and F<ext/>.

Dual-lifed modules maintained by Perl's developers as part of the Perl
core now live in F<dist/>.  Dual-lifed modules maintained primarily on
CPAN now live in F<cpan/>.  When reporting a bug in a module located
under F<cpan/>, please send your bug report directly to the module's
bug tracker or author, rather than Perl's bug tracker.

=item *

C<\N{...}> now compiles better, always forces UTF-8 internal representation

Perl's developers have fixed several problems with the recognition of C<\N{...}>
constructs.  As part of this, perl will store any scalar or regex containing
C<\N{I<name>}> or C<\N{U+I<wide hex char>}> in its definition in
UTF-8 format. (This was true previously for all occurences of C<\N{I<name>}>
that did not use a custom translator, but now it's always true.)

=item *

Perl_magic_setmglob now knows about globs, fixing RT #71254.

=item *

C<SVt_RV> no longer exists. RVs are now stored in IVs.

=item *

REGEXPs are now first class.

=item *

C<Perl_vcroak()> now accepts a null first argument. In addition, a full audit
was made of the "not NULL" compiler annotations, and those for several
other internal functions were corrected.

=item *

New macros C<dSAVEDERRNO>, C<dSAVE_ERRNO>, C<SAVE_ERRNO>, C<RESTORE_ERRNO>
have been added to formalise the temporary saving of the C<errno>
variable.

=item *

The function C<Perl_sv_insert_flags> has been added to augment
C<Perl_sv_insert>.

=item *

The function C<Perl_newSV_type(type)> has been added, equivalent to
C<Perl_newSV()> followed by C<Perl_sv_upgrade(type)>.

=item *

The function C<Perl_newSVpvn_flags()> has been added, equivalent to
C<Perl_newSVpvn()> and then performing the action relevant to the flag.

Two flag bits are currently supported.

=over 4

=item *

C<SVf_UTF8> will call C<SvUTF8_on()> for you. (Note that this does not convert an
sequence of ISO 8859-1 characters to UTF-8). A wrapper, C<newSVpvn_utf8()>
is available for this.

=item *

C<SVs_TEMP> now calls C<Perl_sv_2mortal()> on the new SV.

=back

There is also a wrapper that takes constant strings, C<newSVpvs_flags()>.

=item *

The function C<Perl_croak_xs_usage> has been added as a wrapper to
C<Perl_croak>.

=item *

Perl now exports the functions C<PerlIO_find_layer> and C<PerlIO_list_alloc>.

=item *

C<PL_na> has been exterminated from the core code, replaced by local STRLEN
temporaries, or C<*_nolen()> calls. Either approach is faster than C<PL_na>,
which is a pointer dereference into the interpreter structure under ithreads,
and a global variable otherwise.

=item *

C<Perl_mg_free()> used to leave freed memory accessible via C<SvMAGIC()> on
the scalar. It now updates the linked list to remove each piece of magic
as it is freed.

=item *

Under ithreads, the regex in C<PL_reg_curpm> is now reference counted. This
eliminates a lot of hackish workarounds to cope with it not being reference
counted.

=item *

C<Perl_mg_magical()> would sometimes incorrectly turn on C<SvRMAGICAL()>.
This has been fixed.

=item *

The I<public> IV and NV flags are now not set if the string value has
trailing "garbage". This behaviour is consistent with not setting the
public IV or NV flags if the value is out of range for the type.

=item *

Uses of C<Nullav>, C<Nullcv>, C<Nullhv>, C<Nullop>, C<Nullsv> etc have been
replaced by C<NULL> in the core code, and non-dual-life modules, as C<NULL>
is clearer to those unfamiliar with the core code.

=item *

A macro C<MUTABLE_PTR(p)> has been added, which on (non-pedantic) gcc will
not cast away C<const>, returning a C<void *>. Macros C<MUTABLE_SV(av)>,
C<MUTABLE_SV(cv)> etc build on this, casting to C<AV *> etc without
casting away C<const>. This allows proper compile-time auditing of
C<const> correctness in the core, and helped picked up some errors (now
fixed).

=item *

Macros C<mPUSHs()> and C<mXPUSHs()> have been added, for pushing SVs on the
stack and mortalizing them.

=item *

Use of the private structure C<mro_meta> has changed slightly. Nothing
outside the core should be accessing this directly anyway.

=item *

A new tool, F<Porting/expand-macro.pl> has been added, that allows you
to view how a C preprocessor macro would be expanded when compiled.
This is handy when trying to decode the macro hell that is the perl
guts.

=back

=head1 Testing

=head2 Testing improvements

=over 4

=item Parallel tests

The core distribution can now run its regression tests in parallel on
Unix-like platforms. Instead of running C<make test>, set C<TEST_JOBS> in
your environment to the number of tests to run in parallel, and run
C<make test_harness>. On a Bourne-like shell, this can be done as

    TEST_JOBS=3 make test_harness  # Run 3 tests in parallel

An environment variable is used, rather than parallel make itself, because
L<TAP::Harness> needs to be able to schedule individual non-conflicting test
scripts itself, and there is no standard interface to C<make> utilities to
interact with their job schedulers.

Note that currently some test scripts may fail when run in parallel (most
notably C<ext/IO/t/io_dir.t>). If necessary run just the failing scripts
again sequentially and see if the failures go away.

=item Test harness flexibility

It's now possible to override C<PERL5OPT> and friends in F<t/TEST>

=item Test watchdog

Several tests that have the potential to hang forever if they fail now
incorporate a "watchdog" functionality that will kill them after a timeout,
which helps ensure that C<make test> and C<make test_harness> run to
completion automatically. (Jerry Hedden).


=back

=head2 New Tests

Perl's developers have added a number of new tests to the core.
In addition to the items listed below, many modules updated from CPAN
incorporate new tests.

=over 4

=item *

Significant cleanups to core tests to ensure that language and
interpreter features are not used before they're tested.

=item *

C<make test_porting> now runs a number of important pre-commit checks
which might be of use to anyone working on the Perl core.

=item *

F<t/porting/podcheck.t> automatically checks the well-formedness of
POD found in all .pl, .pm and .pod files in the F<MANIFEST>, other than in
dual-lifed modules which are primarily maintained outside the Perl core.

=item *

F<t/porting/manifest.t> now tests that all files listed in MANIFEST are present.

=item *

F<t/op/while_readdir.t> tests that a bare readdir in while loop sets $_.

=item *

F<t/comp/retainedlines.t> checks that the debugger can retain source lines from C<eval>.

=item *

F<t/io/perlio_fail.t> checks that bad layers fail.

=item *

F<t/io/perlio_leaks.t> checks that PerlIO layers are not leaking.

=item *

F<t/io/perlio_open.t> checks that certain special forms of open work.

=item *

F<t/io/perlio.t> includes general PerlIO tests.

=item *

F<t/io/pvbm.t> checks that there is no unexpected interaction between the internal types
C<PVBM> and C<PVGV>.

=item *

F<t/mro/package_aliases.t> checks that mro works properly in the presence of aliased packages.

=item *

F<t/op/dbm.t> tests C<dbmopen> and C<dbmclose>.

=item *

F<t/op/index_thr.t> tests the interaction of C<index> and threads.

=item *

F<t/op/pat_thr.t> tests the interaction of esoteric patterns and threads.

=item *

F<t/op/qr_gc.t> tests that C<qr> doesn't leak.

=item *

F<t/op/reg_email_thr.t> tests the interaction of regex recursion and threads.

=item *

F<t/op/regexp_qr_embed_thr.t> tests the interaction of patterns with embedded C<qr//> and threads.

=item *

F<t/op/regexp_unicode_prop.t> tests Unicode properties in regular expressions.

=item *

F<t/op/regexp_unicode_prop_thr.t> tests the interaction of Unicode properties and threads.

=item *

F<t/op/reg_nc_tie.t> tests the tied methods of C<Tie::Hash::NamedCapture>.

=item *

F<t/op/reg_posixcc.t> checks that POSIX character classes behave consistently.

=item *

F<t/op/re.t>

checks that exportable C<re> functions in F<universal.c> work.

=item *

F<t/op/setpgrpstack.t> checks that C<setpgrp> works.

=item *

F<t/op/substr_thr.t> tests the interaction of C<substr> and threads.

=item *

F<t/op/upgrade.t> checks that upgrading and assigning scalars works.

=item *

F<t/uni/lex_utf8.t> checks that Unicode in the lexer works.

=item *

F<t/uni/tie.t> checks that Unicode and C<tie> work.

=item *

F<t/comp/final_line_num.t> tests whether line numbers are correct at EOF

=item *

F<t/comp/form_scope.t> tests format scoping.

=item *

F<t/comp/line_debug.t> tests whether C<< @{"_<$file"} >> works.

=item *

F<t/op/filetest_t.t> tests if -t file test works.

=item *

F<t/op/qr.t> tests C<qr>.

=item *

F<t/op/utf8cache.t> tests malfunctions of the utf8 cache.

=item *

F<t/re/uniprops.t> test unicodes C<\p{}> regex constructs.

=item *

F<t/op/filehandle.t> tests some suitably portable filetest operators
to check that they work as expected, particularly in the light of some
internal changes made in how filehandles are blessed.

=item *

F<t/op/time_loop.t> tests that unix times greater than C<2**63>, which
can now be handed to C<gmtime> and C<localtime>, do not cause an internal
overflow or an excessively long loop.

=back


=head1 New or Changed Diagnostics

=head2 New Diagnostics

=over

=item *

SV allocation tracing has been added to the diagnostics enabled by C<-Dm>.
The tracing can alternatively output via the C<PERL_MEM_LOG> mechanism, if
that was enabled when the F<perl> binary was compiled.

=item *

Smartmatch resolution tracing has been added as a new diagnostic. Use C<-DM> to
enable it.

=item *

A new debugging flag C<-DB> now dumps subroutine definitions, leaving
C<-Dx> for its original purpose of dumping syntax trees.

=item *

Perl 5.12 provides a number of new diagnostic messages to help you write
better code.  See L<perldiag> for details of these new messages.

=over 4

=item *

C<Bad plugin affecting keyword '%s'>

=item *

C<gmtime(%.0f) too large>

=item *

C<Lexing code attempted to stuff non-Latin-1 character into Latin-1 input>

=item *

C<Lexing code internal error (%s)>

=item *

C<localtime(%.0f) too large>

=item *

C<Overloaded dereference did not return a reference>

=item *

C<Overloaded qr did not return a REGEXP>

=item *

C<Perl_pmflag() is deprecated, and will be removed from the XS API>

=item *

C<lvalue attribute ignored after the subroutine has been defined>

This new warning is issued when one attempts to mark a subroutine as
lvalue after it has been defined.

=item *

Perl now warns you if C<++> or C<--> are unable to change the value because it's
beyond the limit of representation.

This uses a new warnings category: "imprecision".

=item * 

C<lc>, C<uc>, C<lcfirst>, and C<ucfirst> warn when passed undef.

=item *

C<Show constant in "Useless use of a constant in void context">

=item *

C<Prototype after '%s'>

=item *

C<panic: sv_chop %s>

This new fatal error occurs when the C routine C<Perl_sv_chop()> was
passed a position that is not within the scalar's string buffer. This
could be caused by buggy XS code, and at this point recovery is not
possible.


=item *

The fatal error C<Malformed UTF-8 returned by \N> is now produced if the
C<charnames> handler returns malformed UTF-8.

=item *

If an unresolved named character or sequence was encountered when compiling a
regex pattern then the fatal error C<\\N{NAME} must be resolved by the lexer>
is now produced. This can happen, for example, when using a single-quotish
context like C<$re = '\N{SPACE}'; $re;>. See L<perldiag> for more examples of
how the lexer can get bypassed.

=item *

C<Invalid hexadecimal number in \\N{U+...}> is a new fatal error triggered when
the character constant represented by C<...> is not a valid hexadecimal
number.

=item *

The new meaning of C<\N> as C<[^\n]> is not valid in a bracketed character
class, just like C<.> in a character class loses its special meaning, and will
cause the fatal error C<\\N in a character class must be a named character: \\N{...}>.

=item *

The rules on what is legal for the C<...> in C<\N{...}> have been tightened
up so that unless the C<...> begins with an alphabetic character and continues
with a combination of alphanumerics, dashes, spaces, parentheses or colons
then the warning C<Deprecated character(s) in \\N{...} starting at '%s'> is
now issued.

=item *

The warning C<Using just the first characters returned by \N{}> will be
issued if the C<charnames> handler returns a sequence of characters which
exceeds the limit of the number of characters that can be used. The message
will indicate which characters were used and which were discarded.

=back

=back

=head2 Changed Diagnostics

A number of existing diagnostic messages have been improved or corrected:

=over

=item *

A new warning category C<illegalproto> allows finer-grained control of
warnings around function prototypes.

The two warnings:

=over

=item C<Illegal character in prototype for %s : %s>

=item C<Prototype after '%c' for %s : %s>

=back

have been moved from the C<syntax> top-level warnings category into a new
first-level category, C<illegalproto>. These two warnings are currently the
only ones emitted during parsing of an invalid/illegal prototype, so one
can now do

  no warnings 'illegalproto';

to suppress only those, but not other syntax-related warnings. Warnings where
prototypes are changed, ignored, or not met are still in the C<prototype>
category as before. (Matt S. Trout)

=item *

C<Deep recursion on subroutine "%s">

It is now possible to change the depth threshold for this warning from the
default of 100, by recompiling the F<perl> binary, setting the C
pre-processor macro C<PERL_SUB_DEPTH_WARN> to the desired value.

=item *

C<Illegal character in prototype> warning is now more precise
when reporting illegal characters after _

=item *

mro merging error messages are now very similar to those produced by L<Algorithm::C3>.

=item *

Amelioration of the error message "Unrecognized character %s in column %d"

Changes the error message to "Unrecognized character %s; marked by E<lt>--
HERE after %sE<lt>-- HERE near column %d". This should make it a little
simpler to spot and correct the suspicious character.

=item *

Perl now explicitly points to C<$.> when it causes an uninitialized warning for
ranges in scalar context.

=item *

C<split> now warns when called in void context.

=item *

C<printf>-style functions called with too few arguments will now issue the
warning C<"Missing argument in %s"> [perl #71000]

=item *

Perl now properly returns a syntax error instead of segfaulting
if C<each>, C<keys>, or C<values> is used without an argument.

=item *

C<tell()> now fails properly if called without an argument and when no
previous file was read.

C<tell()> now returns C<-1>, and sets errno to C<EBADF>, thus restoring
the 5.8.x behaviour.

=item *

C<overload> no longer implicitly unsets fallback on repeated 'use
overload' lines.

=item *

POSIX::strftime() can now handle Unicode characters in the format string.

=item *

The Windows select() implementation now supports all empty C<fd_set>s
more correctly.

=item *

The C<syntax> category was removed from 5 warnings that should only be in
C<deprecated>.

=item *

Three fatal C<pack>/C<unpack> error messages have been normalized to
C<panic: %s>

=item *

C<Unicode character is illegal> has been rephrased to be more accurate

It now reads C<Unicode non-character is illegal in interchange> and the
perldiag documentation has been expanded a bit.

=item *

Currently, all but the first of the several characters that the C<charnames>
handler may return are discarded when used in a regular expression pattern
bracketed character class. If this happens then the warning C<Using just the
first character returned by \N{} in character class> will be issued.

=item *

The warning C<Missing right brace on \\N{} or unescaped left brace after \\N.
Assuming the latter> will be issued if Perl encounters a C<\N{> but doesn't
find a matching C<}>. In this case Perl doesn't know if it was mistakenly
omitted, or if "match non-newline" followed by "match a C<{>" was desired.
It assumes the latter because that is actually a valid interpretation as
written, unlike the other case.  If you meant the former, you need to add the
matching right brace.  If you did mean the latter, you can silence this
warning by writing instead C<\N\{>.

=item *

C<gmtime> and C<localtime> called with numbers smaller than they can reliably
handle will now issue the warnings C<gmtime(%.0f) too small> and
C<localtime(%.0f) too small>.

=back

The following diagnostic messages have been removed:

=over 4

=item *

C<Runaway format>

=item *

C<Can't locate package %s for the parents of %s>

In general this warning it only got produced in
conjunction with other warnings, and removing it allowed an ISA lookup
optimisation to be added.

=item *

C<v-string in use/require is non-portable>

=back

=head1 Utility Changes

=over 4

=item *

F<h2ph> now looks in C<include-fixed> too, which is a recent addition to gcc's
search path.

=item *

F<h2xs> no longer incorrectly treats enum values like macros (Daniel Burr).
It also now handles C++ style constants (C<//>) properly in enums. (A patch from
Rainer Weikusat was used; Daniel Burr also proposed a similar fix).

=item *

F<perl5db.pl> now supports C<LVALUE> subroutines.  Additionally, the debugger now correctly handles proxy constant subroutines, and subroutine stubs.

=item *

F<perlbug> now uses C<%Module::CoreList::bug_tracker> to print out
upstream bug tracker URLs.  If a user identifies a particular module
as the topic of their bug report and we're able to divine ithe URL for
its upstream bug tracker, perlbug now provide a message to the user
explaining that the core copies the CPAN version directly, and provide
the URL for reporting the bug directly to the upstream author.

F<perlbug> no longer reports "Message sent" when it hasn't actually sent the message

=item *

F<perlthanks> is a new utility for sending non-bug-reports to the
authors and maintainers of Perl. Getting nothing but bug reports can
become a bit demoralising. If Perl 5.12 works well for you, please try
out F<perlthanks>. It will make the developers smile.

=item *

Perl's developers have fixed bugs in F<a2p> having to do with the
C<match()> operator in list context.

=back

=head1 Selected Bug Fixes

=over 4

=item *

U+0FFFF is now a legal character in regular expressions.

=item *

pp_qr now always returns a new regexp SV. Resolves RT #69852.

Instead of returning a(nother) reference to the (pre-compiled) regexp in the
optree, use reg_temp_copy() to create a copy of it, and return a reference to
that. This resolves issues about Regexp::DESTROY not being called in a timely
fashion (the original bug tracked by RT #69852), as well as bugs related to
blessing regexps, and of assigning to regexps, as described in correspondence
added to the ticket.

It transpires that we also need to undo the SvPVX() sharing when ithreads
cloning a Regexp SV, because mother_re is set to NULL, instead of a cloned
copy of the mother_re. This change might fix bugs with regexps and threads in
certain other situations, but as yet neither tests nor bug reports have
indicated any problems, so it might not actually be an edge case that it's
possible to reach.

=item *

Several compilation errors and segfaults when perl was built with C<-Dmad> were fixed.

=item *

Fixes for lexer API changes in 5.11.2 which broke NYTProf's savesrc option.

=item *

C<-t> should only return TRUE for file handles connected to a TTY

The Microsoft C version of C<isatty()> returns TRUE for all
character mode devices, including the F</dev/null>-style "nul"
device and printers like "lpt1".

=item *

Fixed a regression caused by commit fafafbaf which caused a panic during
parameter passing [perl #70171]

=item *

On systems which in-place edits without backup files, -i'*' now works as
the documentation says it does [perl #70802]

=item *

Saving and restoring magic flags no longer loses readonly flag.

=item *

The malformed syntax C<grep EXPR LIST> (note the missing comma) no longer
causes abrupt and total failure.

=item *

Regular expressions compiled with C<qr{}> literals properly set C<$'> when
matching again.

=item *

Using named subroutines with C<sort> should no longer lead to bus errors [perl
#71076]

=item *

Numerous bugfixes catch small issues caused by the recently-added Lexer API.

=item *

Smart match against C<@_> sometimes gave false negatives. [perl #71078]

=item *

C<$@> may now be assigned a read-only value (without error or busting
the stack).

=item *

C<sort> called recursively from within an active comparison subroutine no
longer causes a bus error if run multiple times. [perl #71076]

=item *

Tie::Hash::NamedCapture::* will not abort if passed bad input (RT #71828)

=item *

@_ and $_ no longer leak under threads (RT #34342 and #41138, also
#70602, #70974)

=item *

C<-I> on shebang line now adds directories in front of @INC
as documented, and as does C<-I> when specified on the command-line.

=item *

C<kill> is now fatal when called on non-numeric process identifiers.
Previously, an C<undef> process identifier would be interpreted as a
request to kill process 0, which would terminate the current process
group on POSIX systems. Since process identifiers are always integers,
killing a non-numeric process is now fatal.

=item *

5.10.0 inadvertently disabled an optimisation, which caused a measurable
performance drop in list assignment, such as is often used to assign
function parameters from C<@_>. The optimisation has been re-instated, and
the performance regression fixed. (This fix is also present in 5.10.1)

=item *

Fixed memory leak on C<while (1) { map 1, 1 }> [RT #53038].

=item *

Some potential coredumps in PerlIO fixed [RT #57322,54828].

=item *

The debugger now works with lvalue subroutines.

=item *

The debugger's C<m> command was broken on modules that defined constants
[RT #61222].

=item *

C<crypt> and string complement could return tainted values for untainted
arguments [RT #59998].

=item *

The C<-i>I<.suffix> command-line switch now recreates the file using
restricted permissions, before changing its mode to match the original
file. This eliminates a potential race condition [RT #60904].

=item *

On some Unix systems, the value in C<$?> would not have the top bit set
(C<$? & 128>) even if the child core dumped.

=item *

Under some circumstances, C<$^R> could incorrectly become undefined
[RT #57042].

=item *

In the XS API, various hash functions, when passed a pre-computed hash where
the key is UTF-8, might result in an incorrect lookup.

=item *

XS code including F<XSUB.h> before F<perl.h> gave a compile-time error
[RT #57176].

=item *

C<< $object-E<gt>isa('Foo') >> would report false if the package C<Foo> didn't
exist, even if the object's C<@ISA> contained C<Foo>.

=item *

Various bugs in the new-to 5.10.0 mro code, triggered by manipulating
C<@ISA>, have been found and fixed.

=item *

Bitwise operations on references could crash the interpreter, e.g.
C<$x=\$y; $x |= "foo"> [RT #54956].

=item *

Patterns including alternation might be sensitive to the internal UTF-8
representation, e.g.

    my $byte = chr(192);
    my $utf8 = chr(192); utf8::upgrade($utf8);
    $utf8 =~ /$byte|X}/i;	# failed in 5.10.0

=item *

Within UTF8-encoded Perl source files (i.e. where C<use utf8> is in
effect), double-quoted literal strings could be corrupted where a C<\xNN>,
C<\0NNN> or C<\N{}> is followed by a literal character with ordinal value
greater than 255 [RT #59908].

=item *

C<B::Deparse> failed to correctly deparse various constructs:
C<readpipe STRING> [RT #62428], C<CORE::require(STRING)> [RT #62488],
C<sub foo(_)> [RT #62484].

=item *

Using C<setpgrp> with no arguments could corrupt the perl stack.

=item *

The block form of C<eval> is now specifically trappable by C<Safe> and
C<ops>. Previously it was erroneously treated like string C<eval>.

=item *

In 5.10.0, the two characters C<[~> were sometimes parsed as the smart
match operator (C<~~>) [RT #63854].

=item *

In 5.10.0, the C<*> quantifier in patterns was sometimes treated as
C<{0,32767}> [RT #60034, #60464]. For example, this match would fail:

    ("ab" x 32768) =~ /^(ab)*$/

=item *

C<shmget> was limited to a 32 bit segment size on a 64 bit OS [RT #63924].

=item *

Using C<next> or C<last> to exit a C<given> block no longer produces a
spurious warning like the following:

    Exiting given via last at foo.pl line 123

=item *

On Windows, C<'.\foo'> and C<'..\foo'>  were treated differently than
C<'./foo'> and C<'../foo'> by C<do> and C<require> [RT #63492].

=item *

Assigning a format to a glob could corrupt the format; e.g.:

     *bar=*foo{FORMAT}; # foo format now bad

=item *

Attempting to coerce a typeglob to a string or number could cause an
assertion failure. The correct error message is now generated,
C<Can't coerce GLOB to I<$type>>.

=item *

Under C<use filetest 'access'>, C<-x> was using the wrong access mode. This
has been fixed [RT #49003].

=item *

C<length> on a tied scalar that returned a Unicode value would not be
correct the first time. This has been fixed.

=item *

Using an array C<tie> inside in array C<tie> could SEGV. This has been
fixed. [RT #51636]

=item *

A race condition inside C<PerlIOStdio_close()> has been identified and
fixed. This used to cause various threading issues, including SEGVs.

=item *

In C<unpack>, the use of C<()> groups in scalar context was internally
placing a list on the interpreter's stack, which manifested in various
ways, including SEGVs. This is now fixed [RT #50256].

=item *

Magic was called twice in C<substr>, C<\&$x>, C<tie $x, $m> and C<chop>.
These have all been fixed.

=item *

A 5.10.0 optimisation to clear the temporary stack within the implicit
loop of C<s///ge> has been reverted, as it turned out to be the cause of
obscure bugs in seemingly unrelated parts of the interpreter [commit
ef0d4e17921ee3de].

=item *

The line numbers for warnings inside C<elsif> are now correct.

=item *

The C<..> operator now works correctly with ranges whose ends are at or
close to the values of the smallest and largest integers.

=item *

C<binmode STDIN, ':raw'> could lead to segmentation faults on some platforms.
This has been fixed [RT #54828].

=item *

An off-by-one error meant that C<index $str, ...> was effectively being
executed as C<index "$str\0", ...>. This has been fixed [RT #53746].

=item *

Various leaks associated with named captures in regexes have been fixed
[RT #57024].

=item *

A weak reference to a hash would leak. This was affecting C<DBI>
[RT #56908].

=item *

Using (?|) in a regex could cause a segfault [RT #59734].

=item *

Use of a UTF-8 C<tr//> within a closure could cause a segfault [RT #61520].

=item *

Calling C<Perl_sv_chop()> or otherwise upgrading an SV could result in an
unaligned 64-bit access on the SPARC architecture [RT #60574].

=item *

In the 5.10.0 release, C<inc_version_list> would incorrectly list
C<5.10.*> after C<5.8.*>; this affected the C<@INC> search order
[RT #67628].

=item *

In 5.10.0, C<pack "a*", $tainted_value> returned a non-tainted value
[RT #52552].

=item *

In 5.10.0, C<printf> and C<sprintf> could produce the fatal error
C<panic: utf8_mg_pos_cache_update> when printing UTF-8 strings
[RT #62666].

=item *

In the 5.10.0 release, a dynamically created C<AUTOLOAD> method might be
missed (method cache issue) [RT #60220,60232].

=item *

In the 5.10.0 release, a combination of C<use feature> and C<//ee> could
cause a memory leak [RT #63110].

=item *

C<-C> on the shebang (C<#!>) line is once more permitted if it is also
specified on the command line. C<-C> on the shebang line used to be a
silent no-op I<if> it was not also on the command line, so perl 5.10.0
disallowed it, which broke some scripts. Now perl checks whether it is
also on the command line and only dies if it is not [RT #67880].

=item *

In 5.10.0, certain types of re-entrant regular expression could crash,
or cause the following assertion failure [RT #60508]:

    Assertion rx->sublen >= (s - rx->subbeg) + i failed

=item *

Perl now includes previously missing files from the Unicode 5.1 Character Database.

=item *

Perl now honors C<TMPDIR> when opening an anonymous temporary file.

=back


=head1 Platform Specific Changes

Perl is incredibly portable. In general, if a platform has a C compiler,
someone has ported Perl to it (or will soon).  We're happy to announce
that Perl 5.12 includes support for several new platforms.  At the same
time, it's time to bid farewell to some (very) old friends.

=head2 New Platforms

=over

=item Haiku

Perl's developers have merged patches from Haiku's maintainers. Perl should now
build on Haiku.

=item MirOS BSD

Perl should now build on MirOS BSD.

=back

=head2 Discontinued Platforms


=over

=item DomainOS

=item Tenon MachTen

=item MiNT

=back

=head2 Updated Platforms

=over 4

=item Darwin (Mac OS X)

=over 4

=item *

Skip testing the be_BY.CP1131 locale on Darwin 10 (Mac OS X 10.6),
as it's still buggy.

=item *

Correct infelicities in the regexp used to identify buggy locales
on Darwin 8 and 9 (Mac OS X 10.4 and 10.5, respectively).

=back

=item DragonFly BSD

=over 4

=item *

Fix thread library selection [perl #69686]

=back

=item Win32

=over 4

=item *

Initial support for mingw64 is now available.

=item *

Various bits of Perl's build infrastructure are no longer converted to
win32 line endings at release time. If this hurts you, please report the
problem with the L<perlbug> program included with perl.

=item *

Always add a manifest resource to C<perl.exe> to specify the C<trustInfo>
settings for Windows Vista and later. Without this setting Windows
will treat C<perl.exe> as a legacy application and apply various
heuristics like redirecting access to protected file system areas
(like the "Program Files" folder) to the users "VirtualStore"
instead of generating a proper "permission denied" error.

For VC8 and VC9 this manifest setting is automatically generated by
the compiler/linker (together with the binding information for their
respective runtime libraries); for all other compilers we need to
embed the manifest resource explicitly in the external resource file.

This change also requests the Microsoft Common-Controls version 6.0
(themed controls introduced in Windows XP) via the dependency list
in the assembly manifest. For VC8 and VC9 this is specified using the
C</manifestdependency> linker commandline option instead.

=item *

Improved message window handling means that C<alarm> and C<kill> messages
will no longer be dropped under race conditions.

=back

=item cygwin

=over 4

=item *

Perl now suppoorts IPv6 on cygwin 1.7 and newer.

=back

=item OpenVMS

=over 4

=item *

C<-UDEBUGGING> is now the default on VMS.

Like it has been everywhere else for ages and ages. Also make
command-line selection of -UDEBUGGING and -DDEBUGGING work in
configure.com; before the only way to turn it off was by saying
no in answer to the interactive question.

=item *

The default pipe buffer size on VMS has been updated to 8192 on 64-bit
systems.

=item *

Reads from the in-memory temporary files of C<PerlIO::scalar> used to fail
if C<$/> was set to a numeric reference (to indicate record-style reads).
This is now fixed.

=item *

VMS now supports C<getgrgid>.

=item *

Many improvements and cleanups have been made to the VMS file name handling
and conversion code.

=item *

Enabling the C<PERL_VMS_POSIX_EXIT> logical name now encodes a POSIX exit
status in a VMS condition value for better interaction with GNV's bash
shell and other utilities that depend on POSIX exit values. See
L<perlvms/"$?"> for details.

=item *

C<File::Copy> now detects Unix compatibility mode on VMS.

=back

=item AIX

Removed F<libbsd> for AIX 5L and 6.1. Only C<flock()> was used from F<libbsd>.

Removed F<libgdbm> for AIX 5L and 6.1. The F<libgdbm> is delivered as an
optional package with the AIX Toolbox. Unfortunately the 64 bit version
is broken.

Hints changes mean that AIX 4.2 should work again.

=item Cygwin

On Cygwin we now strip the last number from the DLL. This has been the
behaviour in the cygwin.com build for years. The hints files have been
updated.


=item FreeBSD

The hints files now identify the correct threading libraries on FreeBSD 7
and later.

=item Irix

We now work around a bizarre preprocessor bug in the Irix 6.5 compiler:
C<cc -E -> unfortunately goes into K&R mode, but C<cc -E file.c> doesn't.

=item NetBSD

Hints now supports versions 5.*.

=item Stratus VOS

Various changes from Stratus have been merged in.

=item Symbian

There is now support for Symbian S60 3.2 SDK and S60 5.0 SDK.

=back


=head1 Known Problems

This is a list of some significant unfixed bugs, which are regressions
from either 5.10.x or 5.8.x.

=over 4

=item *

C<List::Util::first> misbehaves in the presence of a lexical C<$_>
(typically introduced by C<my $_> or implicitly by C<given>). The variable
which gets set for each iteration is the package variable C<$_>, not the
lexical C<$_> [RT #67694].

A similar issue may occur in other modules that provide functions which
take a block as their first argument, like

    foo { ... $_ ...} list

=item *

Some regexes may run much more slowly when run in a child thread compared
with the thread the pattern was compiled into [RT #55600].

=item *

Things like C<"\N{LATIN SMALL LIGATURE FF}" =~ /\N{LATIN SMALL LETTER F}+/>
will appear to hang as they get into a very long running loop [RT #72998].

=item *

Several porters have reported mysterious crashes when Perl's entire test suite is run after a build on certain Windows 2000 systems. When run by hand, the individual tests reportedly work fine.

=back

=head1 Errata

=over

=item *

This one is actually a change introduced in 5.10.0, but it was missed
from that release's perldelta, so it is mentioned here instead.

A bugfix related to the handling of the C</m> modifier and C<qr> resulted
in a change of behaviour between 5.8.x and 5.10.0:

    # matches in 5.8.x, doesn't match in 5.10.0
    $re = qr/^bar/; "foo\nbar" =~ /$re/m;

=back

=head1 Acknowledgements

Perl 5.12.0 represents approximately two years of development since
Perl 5.10.0 and contains over 750,000 lines of changes across over
3000 files from over 200 authors and committers.

Perl continues to flourish into its third decade thanks to a vibrant community of users and developers.  The following people are known to have contributed the improvements that became Perl 5.12.0:

Aaron Crane, Abe Timmerman, Abhijit Menon-Sen, Abigail, Adam Russell,
Adriano Ferreira, Ævar Arnfjörð Bjarmason, Alan Grover, Alexandr
Ciornii, Alex Davies, Alex Vandiver, Andreas Koenig, Andrew Rodland,
andrew@sundale.net, Andy Armstrong, Andy Dougherty, Jose AUGUSTE-ETIENNE,
Benjamin Smith, Ben Morrow, bharanee rathna, Bo Borgerson, Bo Lindbergh,
Brad Gilbert, Bram, Brendan O'Dea, brian d foy, Charles Bailey,
Chip Salzenberg, Chris 'BinGOs' Williams, Christoph Lamprecht, Chris
Williams, chromatic, Claes Jakobsson, Craig A. Berry, Dan Dascalescu,
Daniel Frederick Crisman, Daniel M. Quinlan, Dan Jacobson, Dan Kogai,
Dave Mitchell, Dave Rolsky, David Cantrell, David Dick, David Golden,
David Mitchell, David M. Syzdek, David Nicol, David Wheeler, Dennis
Kaarsemaker, Dintelmann, Peter, Dominic Dunlop, Dr.Ruud, Duke Leto,
Enrico Sorcinelli, Eric Brine, Father Chrysostomos, Florian Ragwitz,
Frank Wiegand, Gabor Szabo, Gene Sullivan, Geoffrey T. Dairiki, George
Greer, Gerard Goossen, Gisle Aas, Goro Fuji, Graham Barr, Green, Paul,
Hans Dieter Pearcey, Harmen, H. Merijn Brand, Hugo van der Sanden,
Ian Goodacre, Igor Sutton, Ingo Weinhold, James Bence, James Mastros,
Jan Dubois, Jari Aalto, Jarkko Hietaniemi, Jay Hannah, Jerry Hedden,
Jesse Vincent, Jim Cromie, Jody Belka, John E. Malmberg, John Malmberg,
John Peacock, John Peacock via RT, John P. Linderman, John Wright,
Josh ben Jore, Jos I. Boumans, Karl Williamson, Kenichi Ishigaki, Ken
Williams, Kevin Brintnall, Kevin Ryde, Kurt Starsinic, Leon Brocard,
Lubomir Rintel, Luke Ross, Marcel Grünauer, Marcus Holland-Moritz, Mark
Jason Dominus, Marko Asplund, Martin Hasch, Mashrab Kuvatov, Matt Kraai,
Matt S Trout, Max Maischein, Michael Breen, Michael Cartmell, Michael
G Schwern, Michael Witten, Mike Giroux, Milosz Tanski, Moritz Lenz,
Nicholas Clark, Nick Cleaton, Niko Tyni, Offer Kaye, Osvaldo Villalon,
Paul Fenwick, Paul Gaborit, Paul Green, Paul Johnson, Paul Marquess,
Philip Hazel, Philippe Bruhat, Rafael Garcia-Suarez, Rainer Tammer,
Rajesh Mandalemula, Reini Urban, Renée Bäcker, Ricardo Signes,
Ricardo SIGNES, Richard Foley, Rich Rauenzahn, Rick Delaney, Risto
Kankkunen, Robert May, Roberto C. Sanchez, Robin Barker, SADAHIRO
Tomoyuki, Salvador Ortiz Garcia, Sam Vilain, Scott Lanning, Sébastien
Aperghis-Tramoni, Sérgio Durigan Júnior, Shlomi Fish, Simon 'corecode'
Schubert, Sisyphus, Slaven Rezic, Smylers, Steffen Müller, Steffen
Ullrich, Stepan Kasal, Steve Hay, Steven Schubiger, Steve Peters, Tels,
The Doctor, Tim Bunce, Tim Jenness, Todd Rinaldo, Tom Christiansen,
Tom Hukins, Tom Wyant, Tony Cook, Torsten Schoenfeld, Tye McQueen,
Vadim Konovalov, Vincent Pit, Hio YAMASHINA, Yasuhiro Matsumoto,
Yitzchak Scott-Thoennes, Yuval Kogman, Yves Orton, Zefram, Zsban Ambrus

This is woefully incomplete as it's automatically generated from version
control history.  In particular, it doesn't include the names of the
(very much appreciated) contributors who reported issues in previous
versions of Perl that helped make Perl 5.12.0 better. For a more complete
list of all of Perl's historical contributors, please see the C<AUTHORS>
file in the Perl 5.12.0 distribution.

Many of the changes included in this version originated in the CPAN
modules included in Perl's core. We're grateful to the entire CPAN
community for helping Perl to flourish.

=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 L<http://rt.perl.org/perlbug/>. There may also be
information at L<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
analyzed by the Perl porting team.

If the bug you are reporting has security implications, which make it
inappropriate to send to a publicly archived mailing list, then please send
it to perl5-security-report@perl.org. This points to a closed subscription
unarchived mailing list, which includes all the core committers, who be able
to help assess the impact of issues, figure out a resolution, and help
co-ordinate the release of patches to mitigate or fix the problem across all
platforms on which Perl is supported. Please only use this address for
security issues in the Perl core, not for modules independently
distributed on CPAN.

=head1 SEE ALSO

The F<Changes> file for an explanation of how to view 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