perldelta: Rearrange the debugger bug section

[perl5.git] / pod / perldelta.pod

=encoding utf8

=head1 NAME

perldelta - what is new for perl v5.14.0

=head1 DESCRIPTION

This document describes differences between the 5.12.0 release and
the 5.14.0 release.

Some of the bug fixes in this release have been backported to subsequent
releases of 5.12.x. Those are indicated with the 5.12.x version in
parentheses.

XXX Go through the perl512*delta files and do that.

=head1 Notice

XXX Any important notices here

=head1 Core Enhancements

=head2 Unicode

=head3 Unicode Version 6.0 is now supported (mostly)

Perl comes with the Unicode 6.0 data base updated with
L<Corrigendum #8|http://www.unicode.org/versions/corrigendum8.html>,
with one exception noted below.
See L<http://unicode.org/versions/Unicode6.0.0> for details on the new
release.  Perl does not support any Unicode provisional properties,
including the new ones for this release, but their database files are
packaged with Perl.

Unicode 6.0 has chosen to use the name C<BELL> for the character at U+1F514,
which is a symbol that looks like a bell, and is used in Japanese cell
phones.  This conflicts with the long-standing Perl usage of having
C<BELL> mean the ASCII C<BEL> character, U+0007.  In Perl 5.14,
C<\N{BELL}> will continue to mean U+0007, but its use will generate a
deprecated warning message, unless such warnings are turned off.  The
new name for U+0007 in Perl will be C<ALERT>, which corresponds nicely
with the existing shorthand sequence for it, C<"\a">.  C<\N{BEL}> will
mean U+0007, with no warning given.  The character at U+1F514 will not
have a name in 5.14, but can be referred to by C<\N{U+1F514}>.  The plan
is that in Perl 5.16, C<\N{BELL}> will refer to U+1F514, and so all code
that uses C<\N{BELL}> should convert by then to using C<\N{ALERT}>,
C<\N{BEL}>, or C<"\a"> instead.

=head3 Full functionality for C<use feature 'unicode_strings'>

This release provides full functionality for C<use feature
'unicode_strings'>.  Under its scope, all string operations executed and
regular expressions compiled (even if executed outside its scope) have
Unicode semantics.  See L<feature>.

This feature avoids most forms of the "Unicode Bug" (See
L<perlunicode/The "Unicode Bug"> for details.)  If there is a
possibility that your code will process Unicode strings, you are
B<strongly> encouraged to use this subpragma to avoid nasty surprises.

=head3 C<\N{I<name>}> and C<charnames> enhancements

=over

=item *

C<\N{}> and C<charnames::vianame> now know about the abbreviated
character names listed by Unicode, such as NBSP, SHY, LRO, ZWJ, etc., all
the customary abbreviations for the C0 and C1 control characters (such as
ACK, BEL, CAN, etc.), and a few new variants of some C1 full names that
are in common usage.

=item *

Unicode has a number of named character sequences, in which particular sequences
of code points are given names.  C<\N{...}> now recognizes these.

=item *

C<\N{}>, C<charnames::vianame>, C<charnames::viacode> now know about every
character in Unicode.  Previously, they didn't know about the Hangul syllables
nor a number of CJK (Chinese/Japanese/Korean) characters.

=item *

In the past, it was ineffective to override one of Perl's abbreviations
with your own custom alias.  Now it works.

=item *

You can also create a custom alias of the ordinal of a
character, known by C<\N{...}>, C<charnames::vianame()>, and
C<charnames::viacode()>.  Previously, an alias had to be to an official
Unicode character name.  This made it impossible to create an alias for
a code point that had no name, such as those reserved for private
use.

=item *

A new function, C<charnames::string_vianame()>, has been added.
This function is a run-time version of C<\N{...}>, returning the string
of characters whose Unicode name is its parameter.  It can handle
Unicode named character sequences, whereas the pre-existing
C<charnames::vianame()> cannot, as the latter returns a single code
point.

=back

See L<charnames> for details on all these changes.

=head3 Any unsigned value can be encoded as a character

With this release, Perl is adopting a model that any unsigned value can
be treated as a code point and encoded internally (as utf8) without
warnings - not just the code points that are legal in Unicode.
However, unless utf8 warnings have been
explicitly lexically turned off, outputting or performing a
Unicode-defined operation (such as upper-casing) on such a code point
will generate a warning.  Attempting to input these using strict rules
(such as with the C<:encoding('UTF-8')> layer) will continue to fail.
Prior to this release the handling was very inconsistent, and incorrect
in places.  Also, the Unicode non-characters, some of which previously were
erroneously considered illegal in places by Perl, contrary to the Unicode
standard, are now always legal internally.  But inputting or outputting
them will work the same as for the non-legal Unicode code points, as the
Unicode standard says they are illegal for "open interchange".

=head3 New warnings categories for problematic (non-)Unicode code points.

Three new warnings subcategories of "utf8" have been added.  These
allow you to turn off some "utf8" warnings, while allowing
others warnings to remain on.  The three categories are:
C<surrogate> when UTF-16 surrogates are encountered;
C<nonchar> when Unicode non-character code points are encountered;
and C<non_unicode> when code points that are above the legal Unicode
maximum of 0x10FFFF are encountered.

=head2 Regular Expressions

=head3 C<(?^...)> construct to signify default modifiers

An ASCII caret (also called a "circumflex accent") C<"^">
immediately following a C<"(?"> in a regular expression
now means that the subexpression does not inherit the
surrounding modifiers such as C</i>, but reverts to the
Perl defaults.  Any modifiers following the caret override the defaults.

The stringification of regular expressions now uses this
notation.  E.g., before, C<qr/hlagh/i> would be stringified as
C<(?i-xsm:hlagh)>, but now it's stringified as C<(?^i:hlagh)>.

The main purpose of this is to allow tests that rely on the
stringification not to have to change when new modifiers are added.
See L<perlre/Extended Patterns>.

=head3 C</d>, C</l>, C</u>, C</a>, and C</aa> modifiers

Four new regular expression modifiers have been added. These are mutually
exclusive; one only can be turned on at a time.

The C</l> modifier says to compile the regular expression as if it were
in the scope of C<use locale>, even if it is not.

The C</u> modifier says to compile the regular expression as if it were
in the scope of a C<use feature "unicode_strings"> pragma.

The C</d> (default) modifier is used to override any C<use locale> and
C<use feature "unicode_strings"> pragmas that are in effect at the time
of compiling the regular expression.

The C</a> regular expression modifier restricts C<\s>, C<\d> and C<\w> and
the Posix (C<[[:posix:]]>) character classes to the ASCII range.  The
complements and C<\b> and C<\B> are correspondingly
affected.  Otherwise, C</a> behaves like the C</u> modifier, in that
case-insensitive matching uses Unicode semantics.

The C</aa> modifier is like C</a>, except that, in case-insensitive matching, no ASCII character will match a
non-ASCII character.  For example,

    'k' =~ /\N{KELVIN SIGN}/ai

will match; it won't under C</aa>.

See L<perlre/Modifiers> for more detail.

=head3 Non-destructive substitution

The substitution (C<s///>) and transliteration
(C<y///>) operators now support an C</r> option that
copies the input variable, carries out the substitution on
the copy and returns the result.  The original remains unmodified.

  my $old = 'cat';
  my $new = $old =~ s/cat/dog/r;
  # $old is 'cat' and $new is 'dog'

This is particularly useful with C<map>.  See L<perlop> for more examples.

=head3 Reentrant regular expression engine

It is now safe to use regular expressions within C<(?{...})> and
C<(??{...})> code blocks inside regular expressions.

These block are still experimental, however, and still have problems with
lexical (C<my>) variables and abnormal exiting.

=head3 C<use re '/flags';>

The C<re> pragma now has the ability to turn on regular expression flags
till the end of the lexical scope:

    use re '/x';
    "foo" =~ / (.+) /;  # /x implied

See L<re/"'/flags' mode"> for details.

=head3 \o{...} for octals

There is a new octal escape sequence, C<"\o">, in double-quote-like
contexts.  This construct allows large octal ordinals beyond the
current max of 0777 to be represented.  It also allows you to specify a
character in octal which can safely be concatenated with other regex
snippets and which won't be confused with being a backreference to
a regex capture group.  See L<perlre/Capture groups>.

=head3 Add C<\p{Titlecase}> as a synonym for C<\p{Title}>

This synonym is added for symmetry with the Unicode property names
C<\p{Uppercase}> and C<\p{Lowercase}>.

=head3 Regular expression debugging output improvement

Regular expression debugging output (turned on by C<use re 'debug';>) now
uses hexadecimal when escaping non-ASCII characters, instead of octal.

=head3 Return value of C<delete $+{...}>

Custom regular expression engines can now determine the return value of
C<delete> on an entry of C<%+> or C<%->.

=head2 Syntactical Enhancements

=head3 Array and hash container functions accept references

All built-in functions that operate directly on array or hash
containers now also accept hard references to arrays or hashes:

  |----------------------------+---------------------------|
  | Traditional syntax         | Terse syntax              |
  |----------------------------+---------------------------|
  | push @$arrayref, @stuff    | push $arrayref, @stuff    |
  | unshift @$arrayref, @stuff | unshift $arrayref, @stuff |
  | pop @$arrayref             | pop $arrayref             |
  | shift @$arrayref           | shift $arrayref           |
  | splice @$arrayref, 0, 2    | splice $arrayref, 0, 2    |
  | keys %$hashref             | keys $hashref             |
  | keys @$arrayref            | keys $arrayref            |
  | values %$hashref           | values $hashref           |
  | values @$arrayref          | values $arrayref          |
  | ($k,$v) = each %$hashref   | ($k,$v) = each $hashref   |
  | ($k,$v) = each @$arrayref  | ($k,$v) = each $arrayref  |
  |----------------------------+---------------------------|

This allows these built-in functions to act on long dereferencing chains
or on the return value of subroutines without needing to wrap them in
C<@{}> or C<%{}>:

  push @{$obj->tags}, $new_tag;  # old way
  push $obj->tags,    $new_tag;  # new way

  for ( keys %{$hoh->{genres}{artists}} ) {...} # old way 
  for ( keys $hoh->{genres}{artists}    ) {...} # new way 

For C<push>, C<unshift> and C<splice>, the reference will auto-vivify
if it is not defined, just as if it were wrapped with C<@{}>.

For C<keys>, C<values>, C<each>, when overloaded dereferencing is
present, the overloaded dereference is used instead of dereferencing the
underlying reftype.  Warnings are issued about assumptions made in
ambiguous cases.

=head3 Single term prototype

The C<+> prototype is a special alternative to C<$> that will act like
C<\[@%]> when given a literal array or hash variable, but will otherwise
force scalar context on the argument.  See L<perlsub/Prototypes>.

=head3 C<package> block syntax

A package declaration can now contain a code block, in which case the
declaration is in scope only inside that block.  So C<package Foo { ... }>
is precisely equivalent to C<{ package Foo; ... }>.  It also works with
a version number in the declaration, as in C<package Foo 1.2 { ... }>.
See L<perlfunc>.

=head3 Statement labels can appear in more places

Statement labels can now occur before any type of statement or declaration,
such as C<package>.

=head3 Stacked labels

Multiple statement labels can now appear before a single statement.

=head3 Uppercase X/B allowed in hexadecimal/binary literals

Literals may now use either upper case C<0X...> or C<0B...> prefixes,
in addition to the already supported C<0x...> and C<0b...>
syntax [perl #76296].

C, Ruby, Python and PHP already supported this syntax, and it makes
Perl more internally consistent. A round-trip with C<eval sprintf
"%#X", 0x10> now returns C<16>, the way C<eval sprintf "%#x", 0x10> does.

=head3 Overridable tie functions

C<tie>, C<tied> and C<untie> can now be overridden [perl #75902].

=head2 Exception Handling

Several changes have been made to the way C<die>, C<warn>, and C<$@>
behave, in order to make them more reliable and consistent.

When an exception is thrown inside an C<eval>, the exception is no
longer at risk of being clobbered by code running during unwinding
(e.g., destructors).  Previously, the exception was written into C<$@>
early in the throwing process, and would be overwritten if C<eval> was
used internally in the destructor for an object that had to be freed
while exiting from the outer C<eval>.  Now the exception is written
into C<$@> last thing before exiting the outer C<eval>, so the code
running immediately thereafter can rely on the value in C<$@> correctly
corresponding to that C<eval>.  (C<$@> is still also set before exiting the
C<eval>, for the sake of destructors that rely on this.)

Likewise, a C<local $@> inside an C<eval> will no longer clobber any
exception thrown in its scope.  Previously, the restoration of C<$@> upon
unwinding would overwrite any exception being thrown.  Now the exception
gets to the C<eval> anyway.  So C<local $@> is safe before a C<die>.

Exceptions thrown from object destructors no longer modify the C<$@>
of the surrounding context.  (If the surrounding context was exception
unwinding, this used to be another way to clobber the exception being
thrown.)  Previously such an exception was
sometimes emitted as a warning, and then either was
string-appended to the surrounding C<$@> or completely replaced the
surrounding C<$@>, depending on whether that exception and the surrounding
C<$@> were strings or objects.  Now, an exception in this situation is
always emitted as a warning, leaving the surrounding C<$@> untouched.
In addition to object destructors, this also affects any function call
performed by XS code using the C<G_KEEPERR> flag.

Warnings for C<warn> can now be objects, in the same way as exceptions
for C<die>.  If an object-based warning gets the default handling,
of writing to standard error, it is stringified as
before, with the file and line number appended.  But
a C<$SIG{__WARN__}> handler will now receive an
object-based warning as an object, where previously it was passed the
result of stringifying the object.

=head2 Other Enhancements

=head3 Assignment to C<$0> sets the legacy process name with C<prctl()> on Linux

On Linux the legacy process name will be set with L<prctl(2)>, in
addition to altering the POSIX name via C<argv[0]> as perl has done
since version 4.000. Now system utilities that read the legacy process
name such as ps, top and killall will recognize the name you set when
assigning to C<$0>. The string you supply will be cut off at 16 bytes,
this is a limitation imposed by Linux.

=head3 C<srand()> now returns the seed

This allows programs that need to have repeatable results not to have to come
up with their own seed-generating mechanism.  Instead, they can use C<srand()>
and stash the return value for future use.  Typical is a test program which
has too many combinations to test comprehensively in the time available to it
each run.  It can test a random subset each time and, should there be a failure,
log the seed used for that run so that it can later be used to reproduce the
same results.

=head3 printf-like functions understand post-1980 size modifiers

Perl's printf and sprintf operators, and Perl's internal printf replacement
function, now understand the C90 size modifiers "hh" (C<char>), "z"
(C<size_t>), and "t" (C<ptrdiff_t>).  Also, when compiled with a C99
compiler, Perl now understands the size modifier "j" (C<intmax_t>).

So, for example, on any modern machine, C<sprintf('%hhd', 257)> returns '1'.

=head3 New global variable C<${^GLOBAL_PHASE}>

A new global variable, C<${^GLOBAL_PHASE}>, has been added to allow
introspection of the current phase of the perl interpreter. It's explained in
detail in L<perlvar/"${^GLOBAL_PHASE}"> and
L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">.

=head3 C<-d:-foo> calls C<Devel::foo::unimport>

The syntax C<-dI<B<:>foo>> was extended in 5.6.1 to make C<-dI<:fooB<=bar>>>
equivalent to C<-MDevel::foo=bar>, which expands
internally to C<use Devel::foo 'bar';>.
F<perl> now allows prefixing the module name with C<->, with the same
semantics as C<-M>, I<i.e.>

=over 4

=item C<-d:-foo>

Equivalent to C<-M-Devel::foo>, expands to
C<no Devel::foo;>, calls C<< Devel::foo->unimport() >>
if the method exists.

=item C<-d:-foo=bar>

Equivalent to C<-M-Devel::foo=bar>, expands to C<no Devel::foo 'bar';>,
calls C<< Devel::foo->unimport('bar') >> if the method exists.

=back

This is particularly useful for suppressing the default actions of a
C<Devel::*> module's C<import> method whilst still loading it for debugging.

=head3 Filehandle method calls load L<IO::File> on demand

When a method call on a filehandle would die because the method cannot
be resolved, and L<IO::File> has not been loaded, Perl now loads L<IO::File>
via C<require> and attempts method resolution again:

  open my $fh, ">", $file;
  $fh->binmode(":raw");     # loads IO::File and succeeds

This also works for globs like STDOUT, STDERR and STDIN:

  STDOUT->autoflush(1);

Because this on-demand load only happens if method resolution fails, the
legacy approach of manually loading an L<IO::File> parent class for partial
method support still works as expected:

  use IO::Handle;
  open my $fh, ">", $file;
  $fh->autoflush(1);        # IO::File not loaded

=head3 IPv6 support

The C<Socket> module provides new affordances for IPv6,
including implementations of the C<Socket::getaddrinfo()> and
C<Socket::getnameinfo()> functions, along with related constants, and a
handful of new functions.  See L<Socket>.

=head3 DTrace probes now include package name

The DTrace probes now include an additional argument (C<arg3>) which contains
the package the subroutine being entered or left was compiled in.

For example using the following DTrace script:

  perl$target:::sub-entry
  {
      printf("%s::%s\n", copyinstr(arg0), copyinstr(arg3));
  }

and then running:

  perl -e'sub test { }; test'

DTrace will print:

  main::test

=head2 New C APIs

See L</Internal Changes>.

=head1 Security

=head2 User-defined regular expression properties

In L<perlunicode/"User-Defined Character Properties">, it says you can
create custom properties by defining subroutines whose names begin with
"In" or "Is". However, Perl did not actually enforce that naming
restriction, so \p{foo::bar} could call foo::bar() if it existed. Now this
convention has been enforced.

Also, Perl no longer allows a tainted regular expression to invoke a
user-defined. It simply dies instead [perl #82616].

=head1 Incompatible Changes

Perl 5.14.0 is not binary-compatible with any previous stable release.

In addition to the sections that follow, see L</C API Changes>.

=head2 Regular Expressions and String Escapes

=head3 C<\cI<X>>

The backslash-c construct was designed as a way of specifying
non-printable characters, but there were no restrictions (on ASCII
platforms) on what the character following the C<c> could be.  Now, that
character must be one of the ASCII characters.

=head3 \400-\777

Use of C<\400>-C<\777> in regexes in certain circumstances has given
different, anomalous behavior than their use in all other
double-quote-like contexts.   Since 5.10.1, a deprecated warning message
has been raised when this happens.  Now, all double-quote-like contexts
have the same behavior, namely to be equivalent to C<\x{100}> -
C<\x{1FF}>, with no deprecation warning. Use of these values in the
command line option C<"-0"> retains the current meaning to slurp input
files whole; previously, this was documented only for C<"-0777">.  It is
recommended, however, because of various ambiguities, to use the new
C<\o{...}> construct to represent characters in octal.

=head3 Most C<\p{}> properties are now immune to case-insensitive matching

For most Unicode properties, it doesn't make sense to have them match
differently under C</i> case-insensitive matching than not.  And doing
so leads to unexpected results and potential security holes.  For
example

 m/\p{ASCII_Hex_Digit}+/i

could previously match non-ASCII characters because of the Unicode
matching rules.  There were a number of bugs in this feature until an
earlier release in the 5.13 series.  Now this release reverts, and
removes the feature completely except for the few properties where
people have come to expect it, namely the ones where casing is an
integral part of their functionality, such as C<m/\p{Uppercase}/i> and
C<m/\p{Lowercase}/i>, both of which match the exact same code points,
namely those matched by C<m/\p{Cased}/i>.  Details are in
L<perlrecharclass/Unicode Properties>.

XXX The mention of ‘until an earlier release in the 5.13 series’ needs to
change, but I do not fully understand what happened here.

User-defined property handlers that need to match differently under
C</i> must change to read the new boolean parameter passed to it which is
non-zero if case-insensitive matching is in effect or 0 otherwise.  See
L<perluniprops/User-Defined Character Properties>.

=head3 \p{} implies Unicode semantics

Now, a Unicode property match specified in the pattern will indicate
that the pattern is meant for matching according to Unicode rules, the way
C<\x{}> does.

=head3 Regular expressions retain their localeness when interpolated

Regular expressions compiled under C<"use locale"> now retain this when
interpolated into a new regular expression compiled outside a
C<"use locale">, and vice-versa.

Previously, a regular expression interpolated into another one inherited
the localeness of the surrounding one, losing whatever state it
originally had.  This is considered a bug fix, but may trip up code that
has come to rely on the incorrect behavior.

=head3 Stringification of regexes has changed

Default regular expression modifiers are now notated by using
C<(?^...)>.  Code relying on the old stringification will fail.  The
purpose of this is so that when new modifiers are added, such code will
not have to change (after this one time), as the stringification will
automatically incorporate the new modifiers.

Code that needs to work properly with both old- and new-style regexes
can avoid the whole issue by using (for Perls since 5.9.5; see L<re>):

 use re qw(regexp_pattern);
 my ($pat, $mods) = regexp_pattern($re_ref);

If the actual stringification is important, or older Perls need to be
supported, you can use something like the following:

    # Accept both old and new-style stringification
    my $modifiers = (qr/foobar/ =~ /\Q(?^/) ? '^' : '-xism';

And then use C<$modifiers> instead of C<-xism>.

=head3 Run-time code blocks in regular expressions inherit pragmata

Code blocks in regular expressions (C<(?{...})> and C<(??{...})>) used not
to inherit any pragmata (strict, warnings, etc.) if the regular expression
was compiled at run time as happens in cases like these two:

  use re 'eval';
  $foo =~ $bar; # when $bar contains (?{...})
  $foo =~ /$bar(?{ $finished = 1 })/;

This was a bug, which has now been fixed. But it has the potential to break
any code that was relying on it.

=head2 Stashes and Package Variables

=head3 Localised tied hashes and arrays are no longed tied

In the following:

    tie @a, ...;
    {
	local @a;
	# here, @a is a now a new, untied array
    }
    # here, @a refers again to the old, tied array

The new local array used to be made tied too, which was fairly pointless,
and has now been fixed. This fix could however potentially cause a change
in behaviour of some code.

=head3 Stashes are now always defined

C<defined %Foo::> now always returns true, even when no symbols have yet been
defined in that package.

This is a side effect of removing a special case kludge in the tokeniser,
added for 5.10.0, to hide side effects of changes to the internal storage of
hashes that drastically reduce their memory usage overhead.

Calling defined on a stash has been deprecated since 5.6.0, warned on
lexicals since 5.6.0, and warned for stashes (and other package
variables) since 5.12.0. C<defined %hash> has always exposed an
implementation detail - emptying a hash by deleting all entries from it does
not make C<defined %hash> false, hence C<defined %hash> is not valid code to
determine whether an arbitrary hash is empty. Instead, use the behaviour
that an empty C<%hash> always returns false in a scalar context.

=head3 Dereferencing typeglobs

If you assign a typeglob to a scalar variable:

    $glob = *foo;

the glob that is copied to C<$glob> is marked with a special flag
indicating that the glob is just a copy. This allows subsequent assignments
to C<$glob> to overwrite the glob. The original glob, however, is
immutable.

Many Perl operators did not distinguish between these two types of globs.
This would result in strange behaviour in edge cases: C<untie $scalar>
would do nothing if the last thing assigned to the scalar was a glob
(because it treated it as C<untie *$scalar>, which unties a handle).
Assignment to a glob slot (e.g., C<*$glob = \@some_array>) would simply
assign C<\@some_array> to C<$glob>.

To fix this, the C<*{}> operator (including the C<*foo> and C<*$foo> forms)
has been modified to make a new immutable glob if its operand is a glob
copy.  This allows operators that make a distinction between globs and
scalars to be modified to treat only immutable globs as globs.  (C<tie>,
C<tied> and C<untie> have been left as they are for compatibility's sake,
but will warn. See L</Deprecations>.)

This causes an incompatible change in code that assigns a glob to the
return value of C<*{}> when that operator was passed a glob copy. Take the
following code, for instance:

    $glob = *foo;
    *$glob = *bar;

The C<*$glob> on the second line returns a new immutable glob. That new
glob is made an alias to C<*bar>. Then it is discarded. So the second
assignment has no effect.

See L<http://rt.perl.org/rt3/Public/Bug/Display.html?id=77810> for even
more detail.

=head3 Clearing stashes

Stash list assignment C<%foo:: = ()> used to make the stash anonymous
temporarily while it was being emptied. Consequently, any of its
subroutines referenced elsewhere would become anonymous (showing up as
"(unknown)" in C<caller>). Now they retain their package names, such that
C<caller> will return the original sub name if there is still a reference
to its typeglob, or "foo::__ANON__" otherwise [perl #79208].

=head3 Magic variables outside the main package

In previous versions of Perl, magic variables like C<$!>, C<%SIG>, etc. would
'leak' into other packages.  So C<%foo::SIG> could be used to access signals,
C<${"foo::!"}> (with strict mode off) to access C's C<errno>, etc.

This was a bug, or an 'unintentional' feature, which caused various ill effects,
such as signal handlers being wiped when modules were loaded, etc.

This has been fixed (or the feature has been removed, depending on how you see
it).

=head2 Changes to Syntax or to Perl Operators

=head3 C<given> return values

C<given> blocks now return the last evaluated
expression, or an empty list if the block was exited by C<break>. Thus you
can now write:

    my $type = do {
     given ($num) {
      break     when undef;
      'integer' when /^[+-]?[0-9]+$/;
      'float'   when /^[+-]?[0-9]+(?:\.[0-9]+)?$/;
      'unknown';
     }
    };

See L<perlsyn/Return value> for details.

=head3 Change in the parsing of certain prototypes

Functions declared with the following prototypes now behave correctly as unary
functions:

  *
  \$ \% \@ \* \&
  \[...]
  ;$ ;*
  ;\$ ;\% etc.
  ;\[...]

Due to this bug fix [perl #75904], functions
using the C<(*)>, C<(;$)> and C<(;*)> prototypes
are parsed with higher precedence than before. So in the following example:

  sub foo($);
  foo $a < $b;

the second line is now parsed correctly as C<< foo($a) < $b >>, rather than
C<< foo($a < $b) >>. This happens when one of these operators is used in
an unparenthesised argument:

  < > <= >= lt gt le ge
  == != <=> eq ne cmp ~~
  &
  | ^
  &&
  || //
  .. ...
  ?:
  = += -= *= etc.

=head3 Smart-matching against array slices

Previously, the following code resulted in a successful match:

    my @a = qw(a y0 z);
    my @b = qw(a x0 z);
    @a[0 .. $#b] ~~ @b;

This odd behaviour has now been fixed [perl #77468].

=head3 Negation treats strings differently from before

The unary negation operator C<-> now treats strings that look like numbers
as numbers [perl #57706].

=head3 Negative zero

Negative zero (-0.0), when converted to a string, now becomes "0" on all
platforms. It used to become "-0" on some, but "0" on others.

If you still need to determine whether a zero is negative, use
C<sprintf("%g", $zero) =~ /^-/> or the L<Data::Float> module on CPAN.

=head3 C<:=> is now a syntax error

Previously C<my $pi := 4;> was exactly equivalent to C<my $pi : = 4;>,
with the C<:> being treated as the start of an attribute list, ending before
the C<=>. The use of C<:=> to mean C<: => was deprecated in 5.12.0, and is now
a syntax error. This will allow the future use of C<:=> as a new token.

We find no Perl 5 code on CPAN using this construction, outside the core's
tests for it, so we believe that this change will have very little impact on
real-world codebases.

If it is absolutely necessary to have empty attribute lists (for example,
because of a code generator) then avoid the error by adding a space before
the C<=>.

=head2 Threads and Processes

=head3 Directory handles not copied to threads

On systems other than Windows that do not have
a C<fchdir> function, newly-created threads no
longer inherit directory handles from their parent threads. Such programs
would usually have crashed anyway [perl #75154].

=head3 C<close> on shared pipes

The C<close> function no longer waits for the child process to exit if the
underlying file descriptor is still in use by another thread, to avoid
deadlocks. It returns true in such cases.

=head3 fork() emulation will not wait for signalled children

On Windows parent processes would not terminate until all forked
childred had terminated first.  However, C<kill('KILL', ...)> is
inherently unstable on pseudo-processes, and C<kill('TERM', ...)>
might not get delivered if the child if blocked in a system call.

To avoid the deadlock and still provide a safe mechanism to terminate
the hosting process, Perl will now no longer wait for children that
have been sent a SIGTERM signal.  It is up to the parent process to
waitpid() for these children if child clean-up processing must be
allowed to finish. However, it is also the responsibility of the
parent then to avoid the deadlock by making sure the child process
can't be blocked on I/O either.

See L<perlfork> for more information about the fork() emulation on
Windows.

=head2 Configuration

=head3 Naming fixes in Policy_sh.SH may invalidate Policy.sh

Several long-standing typos and naming confusions in Policy_sh.SH have
been fixed, standardizing on the variable names used in config.sh.

This will change the behavior of Policy.sh if you happen to have been
accidentally relying on the Policy.sh incorrect behavior.

=head3 Perl source code is read in text mode on Windows

Perl scripts used to be read in binary mode on Windows for the benefit
of the ByteLoader module (which is no longer part of core Perl).  This
had the side effect of breaking various operations on the DATA filehandle,
including seek()/tell(), and even simply reading from DATA after file handles
have been flushed by a call to system(), backticks, fork() etc.

The default build options for Windows have been changed to read Perl source
code on Windows in text mode now.  Hopefully ByteLoader will be updated on
CPAN to automatically handle this situation [perl #28106].

=head1 Deprecations

See also L</Deprecated C APIs>.

=head2 Omitting a space between a regular expression and subsequent word

Omitting a space between a regular expression operator or
its modifiers and the following word is deprecated.  For
example, C<< m/foo/sand $bar >> will still be parsed
as C<< m/foo/s and $bar >> but will issue a warning.

=head2 Deprecation warning added for deprecated-in-core .pl libs

This is a mandatory warning, not obeying -X or lexical warning bits.
The warning is modelled on that supplied by deprecate.pm for
deprecated-in-core .pm libraries.  It points to the specific CPAN
distribution that contains the .pl libraries.  The CPAN version, of
course, does not generate the warning.

=head2 List assignment to C<$[>

Assignment to C<$[> was deprecated and started to give warnings in
Perl version 5.12.0.  This version of perl also starts to emit a warning when
assigning to C<$[> in list context.  This fixes an oversight in 5.12.0.

=head2 Use of qw(...) as parentheses

Historically the parser fooled itself into thinking that C<qw(...)> literals
were always enclosed in parentheses, and as a result you could sometimes omit
parentheses around them:

    for $x qw(a b c) { ... }

The parser no longer lies to itself in this way.  Wrap the list literal in
parentheses, like this:

    for $x (qw(a b c)) { ... }

=head2 C<\N{BELL}> is deprecated

This is because Unicode is using that name for a different character.
See L</Unicode Version 6.0 is now supported (mostly)> for more
explanation.

=head2 C<?PATTERN?> is deprecated

C<?PATTERN?> (without the initial m) has been deprecated and now produces
a warning.  This is to allow future use of C<?> in new operators.
The match-once functionality is still available in the form of C<m?PATTERN?>.

=head2 Tie functions on scalars holding typeglobs

Calling a tie function (C<tie>, C<tied>, C<untie>) with a scalar argument
acts on a file handle if the scalar happens to hold a typeglob.

This is a long-standing bug that will be removed in Perl 5.16, as
there is currently no way to tie the scalar itself when it holds
a typeglob, and no way to untie a scalar that has had a typeglob
assigned to it.

Now there is a deprecation warning whenever a tie
function is used on a handle without an explicit C<*>.

=head2 User-defined case-mapping

This feature is being deprecated due to its many issues, as documented in
L<perlunicode/User-Defined Case Mappings (for serious hackers only)>.
It is planned to remove this feature in Perl 5.16.  A CPAN module
providing improved functionality is being prepared for release by the
time 5.14 is.

XXX What module is that?

=head2 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 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 repercussions 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.

You can silence these deprecation warnings by installing the modules
in question from CPAN.  To install the latest version of all of them,
just install C<Task::Deprecations::5_14>.

=over

=item L<Devel::DProf>

We strongly recommend that you install and used L<Devel::NYTProf> in
preference, as it offers significantly improved profiling and reporting.

=back

=head1 Performance Enhancements

=head2 "Safe signals" optimisation

Signal dispatch has been moved from the runloop into control ops. This
should give a few percent speed increase, and eliminates almost all of
the speed penalty caused by the introduction of "safe signals" in
5.8.0. Signals should still be dispatched within the same statement as
they were previously - if this is not the case, or it is possible to
create uninterruptible loops, this is a bug, and reports are encouraged
of how to recreate such issues.

=head2 Optimisation of shift; and pop; calls without arguments

Two fewer OPs are used for shift and pop calls with no argument (with
implicit C<@_>). This change makes C<shift;> 5% faster than C<shift @_;>
on non-threaded perls and 25% faster on threaded.

=head2 Optimisation of regexp engine string comparison work

The foldEQ_utf8 API function for case-insensitive comparison of strings (which
is used heavily by the regexp engine) was substantially refactored and
optimised - and its documentation much improved as a free bonus gift.

=head2 Regular expression compilation speed-up

Compiling regular expressions has been made faster for the case where upgrading
the regex to utf8 is necessary but that isn't known when the compilation begins.

=head2 String appending is 100 times faster

When doing a lot of string appending, perl could end up allocating a lot more
memory than needed in a very inefficient way, if perl was configured to use the
system's C<malloc> implementation instead of its own.

C<sv_grow>, which is what's being used to allocate more memory if necessary
when appending to a string, has now been taught how to round up the memory
it requests to a certain geometric progression, making it much faster on
certain platforms and configurations.  On Win32, it's now about 100 times
faster.

=head2 Eliminate C<PL_*> accessor functions under ithreads

When C<MULTIPLICITY> was first developed, and interpreter state moved into
an interpreter struct, thread and interpreter local C<PL_*> variables were
defined as macros that called accessor functions, returning the address of
the value, outside of the perl core.  The intent was to allow members
within the interpreter struct to change size without breaking binary
compatibility, so that bug fixes could be merged to a maintenance branch
that necessitated such a size change.

However, some non-core code defines C<PERL_CORE>, sometimes intentionally
to bypass this mechanism for speed reasons, sometimes for other reasons but
with the inadvertent side effect of bypassing this mechanism.  As some of
this code is widespread in production use, the result is that the core
I<can't> change the size of members of the interpreter struct, as it will
break such modules compiled against a previous release on that maintenance
branch.  The upshot is that this mechanism is redundant, and well-behaved
code is penalised by it.  Hence it can and should be removed (and has
been).

=head2 Freeing weak references

When an object has many weak references to it, freeing that object
can under some some circumstances take O(N^2) time to free (where N is the
number of references). The number of circumstances has been reduced
[perl #75254]

=head2 Lexical array and hash assignments

An earlier optimisation to speed up C<my @array = ...> and
C<my %hash = ...> assignments caused a bug and was disabled in Perl 5.12.0.

Now we have found another way to speed up these assignments [perl #82110].

=head2 C<@_> uses less memory

Previously, C<@_> was allocated for every subroutine at compile time with
enough space for four entries.  Now this allocation is done on demand when
the subroutine is called [perl #72416].

=head2 Size optimisations to SV and HV structures

xhv_fill has been eliminated from struct xpvhv, saving 1 IV per hash and
on some systems will cause struct xpvhv to become cache-aligned. To avoid
this memory saving causing a slowdown elsewhere, boolean use of HvFILL
now calls HvTOTALKEYS instead (which is equivalent) - so while the fill
data when actually required are now calculated on demand, the cases when
this needs to be done should be few and far between.

The order of structure elements in SV bodies has changed.  Effectively,
the NV slot has swapped location with STASH and MAGIC.  As all access to
SV members is via macros, this should be completely transparent.  This
change allows the space saving for PVHVs documented above, and may reduce
the memory allocation needed for PVIVs on some architectures.

C<XPV>, C<XPVIV>, and C<XPVNV> now only allocate the parts of the C<SV> body
they actually use, saving some space.

Scalars containing regular expressions now only allocate the part of the C<SV>
body they actually use, saving some space.

=head2 Memory consumption improvements to Exporter

The @EXPORT_FAIL AV is no longer created unless required, hence neither is
the typeglob backing it.  This saves about 200 bytes for every package that
uses Exporter but doesn't use this functionality.

=head2 Memory savings for weak references

For weak references, the common case of just a single weak reference per
referent has been optimised to reduce the storage required. In this case it
saves the equivalent of one small Perl array per referent.

=head2 C<%+> and C<%-> use less memory

The bulk of the C<Tie::Hash::NamedCapture> module used to be in the perl
core.  It has now been moved to an XS module, to reduce the overhead for
programs that do not use C<%+> or C<%->.

=head2 Multiple small improvements to threads

The internal structures of threading now make fewer API calls and fewer
allocations, resulting in noticeably smaller object code.  Additionally,
many thread context checks have been deferred so that they're only done
when required (although this is only possible for non-debugging builds).

=head2 Adjacent pairs of nextstate opcodes are now optimized away

Previously, in code such as

    use constant DEBUG => 0;

    sub GAK {
        warn if DEBUG;
        print "stuff\n";
    }

the ops for C<warn if DEBUG;> would be folded to a C<null> op (C<ex-const>), but
the C<nextstate> op would remain, resulting in a runtime op dispatch of
C<nextstate>, C<nextstate>, ....

The execution of a sequence of C<nextstate> ops is indistinguishable from just
the last C<nextstate> op so the peephole optimizer now eliminates the first of
a pair of C<nextstate> ops, except where the first carries a label, since labels
must not be eliminated by the optimizer and label usage isn't conclusively known
at compile time.

=head1 Modules and Pragmata

=head2 New Modules and Pragmata

=over 4

=item *

C<CPAN::Meta::YAML> 0.003 has been added as a dual-life module.  It supports a
subset of YAML sufficient for reading and writing META.yml and MYMETA.yml files
included with CPAN distributions or generated by the module installation
toolchain. It should not be used for any other general YAML parsing or
generation task.

=item *

C<CPAN::Meta> version 2.110440 has been added as a dual-life module. It
provides a standard library to read, interpret and write CPAN distribution
metadata files (e.g. META.json and META.yml) which describes a
distribution, its contents, and the requirements for building it and
installing it. The latest CPAN distribution metadata specification is
included as C<CPAN::Meta::Spec> and notes on changes in the specification
over time are given in C<CPAN::Meta::History>.

=item *

C<HTTP::Tiny> 0.010 has been added as a dual-life module.  It is a very
small, simple HTTP/1.1 client designed for simple GET requests and file
mirroring.  It has has been added to enable CPAN.pm and CPANPLUS to
"bootstrap" HTTP access to CPAN using pure Perl without relying on external
binaries like F<curl> or F<wget>.

=item *

C<JSON::PP> 2.27105 has been added as a dual-life module, for the sake of
reading F<META.json> files in CPAN distributions.

=item *

C<Module::Metadata> 1.000003 has been added as a dual-life module.  It gathers
package and POD information from Perl module files.  It is a standalone module
based on Module::Build::ModuleInfo for use by other module installation
toolchain components.  Module::Build::ModuleInfo has been deprecated in
favor of this module instead.

=item *

C<Perl::OSType> 1.002 has been added as a dual-life module.  It maps Perl
operating system names (e.g. 'dragonfly' or 'MSWin32') to more generic types
with standardized names (e.g.  "Unix" or "Windows").  It has been refactored
out of Module::Build and ExtUtils::CBuilder and consolidates such mappings into
a single location for easier maintenance.

=item *

The following modules were added by the C<Unicode::Collate> 
upgrade. See below for details.

C<Unicode::Collate::CJK::Big5>

C<Unicode::Collate::CJK::GB2312>

C<Unicode::Collate::CJK::JISX0208>

C<Unicode::Collate::CJK::Korean>

C<Unicode::Collate::CJK::Pinyin>

C<Unicode::Collate::CJK::Stroke>

=item *

C<Version::Requirements> version 0.101020 has been added as a dual-life
module.  It provides a standard library to model and manipulates module
prerequisites and version constraints as defined in the L<CPAN::Meta::Spec>.

=back

=head2 Selected Module and Pragma Updates

=over 4

=item *

C<Archive::Extract> has been upgraded from version 0.38 to 0.48.

Updates since 0.38 include: a safe print method that guards
Archive::Extract from changes to $\; a fix to the tests when run in core
perl; support for TZ files; a modification for the lzma
logic to favour IO::Uncompress::Unlzma; and a fix
for an issue with NetBSD-current and its new unzip 
executable.

=item *

C<Archive::Tar> has been upgraded from version 1.54 to 1.76.

Important changes since 1.54 include the following:

=over

=item *

Compatibility with busybox implementations of tar

=item *

A fix so that C<write()> and C<create_archive()>
close only handles they opened

=item *

A bug was fixed regarding the exit code of extract_archive.

=item *

C<ptar> has a new option to allow safe
creation of tarballs without world-writable files on Windows, allowing those
archives to be uploaded to CPAN.

=item *

A new ptargrep utility for using regular expressions against 
the contents of files in a tar archive.

=item *

Pax extended headers are now skipped.

=back

=item *

C<B> has been upgraded from version 1.23 to 1.27.

It no longer crashes when taking apart a C<y///> containing characters
outside the octet range or compiled in a C<use utf8> scope.

The size of the shared object has been reduced by about 40%, with no
reduction in functionality.

=item *

C<B::Concise> has been upgraded from version 0.78 to 0.82.

B::Concise marks rv2sv, rv2av and rv2hv ops with the new OPpDEREF flag
as "DREFed".

It no longer produces mangled output with the C<-tree> option
[perl #80632].

=item *

C<B::Deparse> has been upgraded from version 0.96 to 1.02.

The deparsing of a nextstate op has changed when it has both a
change of package (relative to the previous nextstate), or a change of
C<%^H> or other state, and a label.  Previously the label was emitted
first, but now the label is emitted last.

The C<no 5.13.2> or similar form is now correctly handled by B::Deparse.

B::Deparse now properly handles the code that applies a conditional
pattern match against implicit C<$_> as it was fixed in [perl #20444].

Deparsing of C<our> followed by a variable with funny characters
(as permitted under the C<utf8> pragma) has also been fixed [perl #33752].

=item *

C<Carp> has been upgraded from version 1.18 to 1.19.

L<Carp> now detects incomplete L<caller()|perlfunc/"caller EXPR"> overrides and
avoids using bogus C<@DB::args>. To provide backtraces, Carp relies on
particular behaviour of the caller built-in. Carp now detects if other code has
overridden this with an incomplete implementation, and modifies its backtrace
accordingly. Previously incomplete overrides would cause incorrect values in
backtraces (best case), or obscure fatal errors (worst case)

This fixes certain cases of C<Bizarre copy of ARRAY> caused by modules
overriding C<caller()> incorrectly.

It now also avoids using regular expressions that cause perl to
load its Unicode tables, in order to avoid the 'BEGIN not safe after
errors' error that will ensue if there has been a syntax error
[perl #82854].

=item *

C<CGI> has been upgraded from version 3.48 to 3.51.

This provides the following security fixes: the MIME boundary in 
multipart_init is now random and the handling of 
newlines embedded in header values has been improved.

=item *

C<Compress::Raw::Bzip2> has been upgraded from version 2.024 to 2.033.

It has been updated to use bzip2 1.0.6.

=item *

C<CPAN> has been upgraded from version 1.94_56 to 1.9600.-

Major highlights:

=over 4

=item * much less configuration dialog hassle

=item * support for META/MYMETA.json

=item * support for local::lib

=item * support for HTTP::Tiny to reduce the dependency on ftp sites 

=item * automatic mirror selection

=item * iron out all known bugs in configure_requires

=item * support for distributions compressed with bzip2

=item * allow Foo/Bar.pm on the commandline to mean Foo::Bar

=back

=item *

C<CPANPLUS> has been upgraded from version 0.90 to 0.9102.

A dependency on Config was not recognised as a
core module dependency.  This has been fixed.

CPANPLUS now includes support for META.json and MYMETA.json.

=item *

C<Data::Dumper> has been upgraded from version 2.125 to 2.130_02.

The indentation used to be off when C<$Data::Dumper::Terse> was set. This
has been fixed [perl #73604].

This upgrade also fixes a crash when using custom sort functions that might
cause the stack to change [perl #74170].

C<Dumpxs> no longer crashes with globs returned by C<*$io_ref>
[perl #72332].

=item *

C<Devel::DProf> has been upgraded from version 20080331.00 to 20110228.00.

Merely loading C<Devel::DProf> now no longer triggers profiling to start.
C<use Devel::DProf> and C<perl -d:DProf ...> still behave as before and start
the profiler.

NOTE: C<Devel::DProf> is deprecated and will be removed from a future
version of Perl.  We strongly recommend that you install and use
L<Devel::NYTProf> instead, as it offers significantly improved
profiling and reporting.

=item *

C<diagnostics> has been upgraded from version 1.19 to 1.22.

It now renders pod links slightly better, and has been taught to find
descriptions for messages that share their descriptions with other
messages.

=item *

C<Digest::MD5> has been upgraded from version 2.39 to 2.51.

It is now safe to use this module in combination with threads.

=item *

C<Digest::SHA> has been upgraded from version 5.47 to 5.61.

C<shasum> now more closely mimics C<sha1sum>/C<md5sum>.

C<Addfile> accepts all POSIX filenames.

New SHA-512/224 and SHA-512/256 transforms (ref. NIST Draft FIPS 180-4
[February 2011])

=item *

C<DynaLoader> has been upgraded from version 1.10 to 1.12.

It fixes a buffer overflow when passed a very long file name.

It no longer inherits from AutoLoader; hence it no longer
produces weird error messages for unsuccessful method calls on classes that
inherit from DynaLoader [perl #84358].

=item *

C<Encode> has been upgraded from version 2.39 to 2.42.

Now, all 66 Unicode non-characters are treated the same way U+FFFF has
always been treated; in cases when it was disallowed, all 66 are
disallowed; in those cases where it warned, all 66 warn.

=item *

C<Errno> has been upgraded from version 1.11 to 1.13.

The implementation of C<Errno> has been refactored to use about 55% less memory.

On some platforms with unusual header files, like Win32/gcc using mingw64
headers, some constants which weren't actually error numbers have been exposed
by C<Errno>.  This has been fixed [perl #77416].

=item *

C<Exporter> has been upgraded from version 5.64_01 to 5.64_03.

Exporter no longer overrides C<$SIG{__WARN__}> [perl #74472]

=item *

C<ExtUtils::Constant> has been upgraded from 0.22 to 0.23.

The C<AUTOLOAD> helper code generated by C<ExtUtils::Constant::ProxySubs>
can now C<croak> for missing constants, or generate a complete C<AUTOLOAD>
subroutine in XS, allowing simplification of many modules that use it
(C<Fcntl>, C<File::Glob>, C<GDBM_File>, C<I18N::Langinfo>, C<POSIX>,
C<Socket>).

C<ExtUtils::Constant::ProxySubs> can now optionally push the names of all
constants onto the package's C<@EXPORT_OK>.

=item *

C<File::DosGlob> has been upgraded from version 1.01 to 1.03.

It allows patterns containing literal parentheses (they no longer need to
be escaped).  On Windows, it no longer
adds an extra F<./> to the file names
returned when the pattern is a relative glob with a drive specification,
like F<c:*.pl> [perl #71712].

=item *

C<File::Fetch> has been upgraded from version 0.24 to 0.32.

C<HTTP::Lite> is now supported for 'http' scheme.

The C<fetch> utility is supported on FreeBSD, NetBSD and
Dragonfly BSD for the C<http> and C<ftp> schemes.

=item *

C<File::Find> has been upgraded from version 1.15 to 1.18.

It improves handling of backslashes on Windows, so that paths like
F<c:\dir\/file> are no longer generated [perl #71710].

=item *

C<File::stat> has been upgraded from 1.02 to 1.04.

The C<-x> and C<-X> file test operators now work correctly under the root
user.

=item *

C<GDBM_File> has been upgraded from 1.10 to 1.13.

This fixes a memory leak when DBM filters are used.

=item *

C<Hash::Util> has been upgraded from 0.07 to 0.10.

Hash::Util no longer emits spurious "uninitialized" warnings when
recursively locking hashes that have undefined values [perl #74280].

=item *

C<I18N::Langinfo> has been upgraded from version 0.03 to 0.07.

C<langinfo()> now defaults to using C<$_> if there is no argument given, just
as the documentation has always claimed.

=item *

C<IO::Select> has been upgraded from version 1.17 to 1.18.

It now allows IO::Handle objects (and objects in derived classes) to be
removed from an IO::Select set even if the underlying file descriptor is
closed or invalid.

=item *

C<IPC::Cmd> has been upgraded from version 0.54 to 0.68.

Resolves an issue with splitting Win32 command lines.  An argument
consisting of the single character "0" used to be omitted (CPAN RT #62961).

=item *

C<IPC::Open3> has been upgraded from 1.05 to 1.08.

C<open3> now produces an error if the C<exec> call fails, allowing this
condition to be distinguished from a child process that exited with a
non-zero status [perl #72016].

The internal C<xclose> routine now knows how to handle file descriptors, as
documented, so duplicating STDIN in a child process using its file
descriptor now works [perl #76474].

=item *

C<Locale::Maketext> has been upgraded from version 1.14 to 1.17.

Locale::Maketext now supports external caches.

This upgrade also fixes an infinite loop in
C<Locale::Maketext::Guts::_compile()> when
working with tainted values (CPAN RT #40727).

C<< ->maketext >> calls will now back up and restore C<$@> so that error
messages are not suppressed (CPAN RT #34182).

=item *

C<Math::BigInt> has been upgraded from version 1.89_01 to 1.994.

This fixes, among other things, incorrect results when computing binomial
coefficients [perl #77640].

It also prevents C<sqrt($int)> from crashing under C<use bigrat;>
[perl #73534].

=item *

C<MIME::Base64> has been upgraded from 3.08 to 3.13.

Includes new functions to calculate the length of encoded and decoded
base64 strings.

Now provides C<encode_base64url> and C<decode_base64url> functions to process
the base64 scheme for "URL applications".

=item *

C<Module::Build> has been upgraded from version 0.3603 to 0.3800.

A notable change is the deprecation of several modules.
Module::Build::Version has been deprecated and Module::Build now relies
directly upon L<version>.  Module::Build::ModuleInfo has been deprecated in
favor of a standalone copy of it called L<Module::Metadata>.
Module::Build::YAML has been deprecated in favor of L<CPAN::Meta::YAML>.

Module::Build now also generates META.json and MYMETA.json files
in accordance with version 2 of the CPAN distribution metadata specification,
L<CPAN::Meta::Spec>.  The older format META.yml and MYMETA.yml files are
still generated, as well.

=item *

C<Module::CoreList> has been upgraded from version 2.29 to XXX.

Besides listing the updated core modules of this release, it also stops listing
the C<Filespec> module.  That module never existed in core.  The scripts
generating C<Module::CoreList> confused it with C<VMS::Filespec>, which actually
is a core module as of perl 5.8.7.

=item *

C<NDBM_File> and C<ODBM_File> have been upgraded from 1.08 to 1.11, and
from 1.08 to 1.09, respectively.

This fixes a memory leak when DBM filters are used.

=item *

C<overload> has been upgraded from 1.11 to 1.12.

C<overload::Method> can now handle subroutines that are themselves blessed
into overloaded classes [perl #71998].

The documentation has greatly improved. See L</Documentation> below.

=item *

C<Parse::CPAN::Meta> has been upgraded from version 1.40 to 1.4401.

The latest Parse::CPAN::Meta can now read YAML and JSON files using
L<CPAN::Meta::YAML> and L<JSON::PP>, which are now part of the Perl core.

=item *

C<PerlIO::scalar> has been upgraded from 0.07 to 0.11.

A C<read> after a C<seek> beyond the end of the string no longer thinks it
has data to read [perl #78716].

=item *

C<POSIX> has been upgraded from 1.19 to 1.23.

It now includes constants for POSIX signal constants.

=item *

C<re> has been upgraded from version 0.11 to 0.17.

New C<use re "/flags"> pragma

The C<regmust> function used to crash when called on a regular expression
belonging to a pluggable engine.  Now it croaks instead.

C<regmust> no longer leaks memory.

=item *

C<Safe> has been upgraded from version 2.25 to 2.29.

This fixes a possible infinite loop when looking for coderefs.

It adds C<&version::vxs::VCMP> to the default share.

=item *

C<SelfLoader> has been upgraded from 1.17 to 1.18.

It now works in taint mode [perl #72062].

=item *

C<sigtrap> has been upgraded from version 1.04 to 1.05.

It no longer tries to modify read-only arguments when generating a
backtrace [perl #72340].

=item *

C<Socket> has been upgraded from version 1.87 to 1.94.

See L</IPv6 support>, above.

=item *

C<Storable> has been upgraded from version 2.22 to 2.27.

Includes performance improvement for overloaded classes.

This adds support for serialising code references that contain UTF-8 strings
correctly.  The Storable minor version
number changed as a result, meaning that
Storable users who set C<$Storable::accept_future_minor> to a C<FALSE> value
will see errors (see L<Storable/FORWARD COMPATIBILITY> for more details).

Freezing no longer gets confused if the Perl stack gets reallocated
during freezing [perl #80074].

=item *

C<Test::Simple> has been upgraded from version 0.94 to 0.98.

Among many other things, subtests without a C<plan> or C<no_plan> now have an
implicit C<done_testing()> added to them.

=item *

C<Thread::Semaphore> has been upgraded from version 2.09 to 2.12.

It provides two new methods that give more control over the decrementing of
semaphores: C<down_nb> and C<down_force>.

=item *

C<Tie::Hash> has been upgraded from version 1.03 to 1.04.

Calling C<< Tie::Hash-E<gt>TIEHASH() >> used to loop forever.  Now it C<croak>s.

=item *

C<Unicode::Collate> has been upgraded from version 0.52_01 to 0.73.

Unicode::Collate has been updated to use Unicode 6.0.0.

Unicode::Collate::Locale now supports a plethora of new locales: ar, be,
bg, de__phonebook, hu, hy, kk, mk, nso, om, tn, vi, hr, ig, ja, ko, ru, sq, 
se, sr, to, uk, zh, zh__big5han, zh__gb2312han, zh__pinyin and zh__stroke.

The following modules have been added:

C<Unicode::Collate::CJK::Big5> for C<zh__big5han> which makes 
tailoring of CJK Unified Ideographs in the order of CLDR's big5han ordering.

C<Unicode::Collate::CJK::GB2312> for C<zh__gb2312han> which makes
tailoring of CJK Unified Ideographs in the order of CLDR's gb2312han ordering.

C<Unicode::Collate::CJK::JISX0208> which makes tailoring of 6355 kanji 
(CJK Unified Ideographs) in the JIS X 0208 order.

C<Unicode::Collate::CJK::Korean> which makes tailoring of CJK Unified Ideographs 
in the order of CLDR's Korean ordering.

C<Unicode::Collate::CJK::Pinyin> for C<zh__pinyin> which makes
tailoring of CJK Unified Ideographs in the order of CLDR's pinyin ordering.

C<Unicode::Collate::CJK::Stroke> for C<zh__stroke> which makes
tailoring of CJK Unified Ideographs in the order of CLDR's stroke ordering.

This also sees the switch from using the pure-perl version of this
module to the XS version.

=item *

C<Unicode::UCD> has been upgraded from version 0.27 to 0.32.

A new function, C<Unicode::UCD::num()>, has been added.  This function
returns the numeric value of the string passed it or C<undef> if the string
in its entirety has no "safe" numeric value.  (For more detail, and for the
definition of "safe", see L<Unicode::UCD/num>.)

This upgrade also includes a number of bug fixes:

=over 4

=item charinfo()

=over 4

=item *

It is now updated to Unicode Version 6 with Corrigendum #8, except,
as with Perl 5.14, the code point at U+1F514 has no name.

=item *

The Hangul syllable code points have the correct names, and their
decompositions are always output without requiring L<Lingua::KO::Hangul::Util>
to be installed.

=item *

The CJK (Chinese-Japanese-Korean) code points U+2A700 to U+2B734
and U+2B740 to U+2B81D are now properly handled.

=item *

The numeric values are now output for those CJK code points that have them.

=item *

The names that are output for code points with multiple aliases are now the
corrected ones.

=back

=item charscript()

This now correctly returns "Unknown" instead of C<undef> for the script
of a code point that hasn't been assigned another one.

=item charblock()

This now correctly returns "No_Block" instead of C<undef> for the block
of a code point that hasn't been assigned to another one.

=back

=item *

C<version> has been upgraded from 0.82 to 0.88.

Due to a bug, now fixed, the C<is_strict> and C<is_lax> functions did not
work when exported.

=item *

C<warnings> and C<warnings::register> have been upgraded from version 1.09
to 1.11 and from version 1.01 to 1.02 respectively.

Calling C<use warnings> without arguments is now significantly more efficient.

It is now possible to register warning categories other than the names of
packages using C<warnings::register>.  See L<perllexwarn> for more information.

=item *

C<VMS::DCLsym> has been upgraded from version 1.03 to 1.05.

Two bugs have been fixed [perl #84086]:

The symbol table name was lost when tying a hash, due to a thinko in
C<TIEHASH>. The result was that all tied hashes interacted with the
local symbol table.

Unless a symbol table name had been explicitly specified in the call
to the constructor, querying the special key ':LOCAL' failed to
identify objects connected to the local symbol table.

=item *

C<Win32> has been upgraded from version 0.39 to 0.44.

This release has several new functions: C<Win32::GetSystemMetrics>,
C<Win32::GetProductInfo>, C<Win32::GetOSDisplayName>.

The names returned by C<Win32::GetOSName> and C<Win32::GetOSDisplayName>
have been corrected.

=back

=head2 Removed Modules and Pragmata

The following modules have been removed from the core distribution, and if
needed should be installed from CPAN instead.

=over

=item C<Class::ISA>

=item C<Pod::Plainer>

=item C<Switch>

=back

The removal of C<Shell> has been deferred until after 5.14, as the
implementation of C<Shell> shipped with 5.12.0 did not correctly issue the
warning that it was to be removed from core.

=head1 Documentation

=head2 New Documentation

=head3 L<perlgpl>

L<perlgpl> has been updated to contain GPL version 1, as is included in the
F<README> distributed with perl.

=head3 Perl 5.12.x delta files

The perldelta files for Perl 5.12.1 to 5.12.3 have been added from the
maintenance branch: L<perl5121delta>, L<perl5122delta>, L<perl5123delta>.

=head3 L<perlpodstyle>

New style guide for POD documentation,
split mostly from the NOTES section of the pod2man man page.

=head3 L<perlsource>, L<perlinterp>, L<perlhacktut>, and L<perlhacktips>

See L</L<perlhack> and perlrepository revamp>, below.

=head2 Changes to Existing Documentation

=head3 L<perlmodlib> is now complete

The perlmodlib page that came with Perl 5.12.0 was missing a lot of
modules, due to a bug in the script that generates the list. This has been
fixed [perl #74332].

=head3 Replace wrong tr/// table in L<perlebcdic>

L<perlebcdic> contains a helpful table to use in tr/// to convert
between EBCDIC and Latin1/ASCII.  Unfortunately, the table was the
inverse of the one it describes, though the code that used the table
worked correctly for the specific example given.

The table has been changed to its inverse, and the sample code changed
to correspond, as this is easier for the person trying to follow the
instructions since deriving the old table is somewhat more complicated.

The table has also been changed to hex from octal, as that is more the norm
these days, and the recipes in the pod altered to print out leading
zeros to make all the values the same length.

=head3 Tricks for user-defined casing

L<perlunicode> now contains an explanation of how to override, mangle
and otherwise tweak the way perl handles upper-, lower- and other-case
conversions on Unicode data, and how to provide scoped changes to alter
one's own code's behaviour without stomping on anybody else.

=head3 INSTALL explicitly states the requirement for C89

This was already true but it's now Officially Stated For The Record.

=head3 Explanation of C<\xI<HH>> and C<\oI<OOO>> escapes

L<perlop> has been updated with more detailed explanation of these two
character escapes.

=head3 C<-0I<NNN>> switch

In L<perlrun>, the behavior of the C<-0NNN> switch for C<-0400> or higher
has been clarified.

=head3 Deprecation policy

L<perlpolicy> now contains the policy on compatibility and deprecation
along with definitions of terms like "deprecation"

=head3 New descriptions in L<perldiag>

The following existing diagnostics are now documented:

=over 4

=item *

L<Ambiguous use of %c resolved as operator %c|perldiag/"Ambiguous use of %c resolved as operator %c">

=item *

L<Ambiguous use of %c{%s} resolved to %c%s|perldiag/"Ambiguous use of %c{%s} resolved to %c%s">

=item *

L<Ambiguous use of %c{%s%s} resolved to %c%s%s|perldiag/"Ambiguous use of %c{%s%s} resolved to %c%s%s">

=item *

L<Ambiguous use of -%s resolved as -&%s()|perldiag/"Ambiguous use of -%s resolved as -&%s()">

=item *

L<Invalid strict version format (%s)|perldiag/"Invalid strict version format (%s)">

=item *

L<Invalid version format (%s)|perldiag/"Invalid version format (%s)">

=item *

L<Invalid version object|perldiag/"Invalid version object">

=back

=head3 L<perlbook>

L<perlbook> has been expanded to cover many more popular books.

=head3 C<SvTRUE> macro

The documentation for the C<SvTRUE> macro in
L<perlapi> was simply wrong in stating that
get-magic is not processed. It has been corrected.

=head3 L<perlvar> revamp

L<perlvar> reorders the variables and groups them by topic.  Each variable
introduced after Perl 5.000 notes the first version in which it is 
available.  L<perlvar> also has a new section for deprecated variables to
note when they were removed.

=head3 Array and hash slices in scalar context

These are now documented in L<perldata>.

=head3 C<use locale> and formats

L<perlform> and L<perllocale> have been corrected to state that
C<use locale> affects formats.

=head3 L<overload>

L<overload>'s documentation has practically undergone a rewrite. It
is now much more straightforward and clear.

=head3 L<perlhack> and perlrepository revamp

The L<perlhack> and perlrepository documents have been heavily edited and
split up into several new documents.

The L<perlhack> document is now much shorter, and focuses on the Perl 5
development process and submitting patches to Perl. The technical content has
been moved to several new documents, L<perlsource>, L<perlinterp>,
L<perlhacktut>, and L<perlhacktips>. This technical content has only been
lightly edited.

The perlrepository document has been renamed to L<perlgit>. This new document
is just a how-to on using git with the Perl source code. Any other content
that used to be in perlrepository has been moved to perlhack.

=head3 Time::Piece examples

Examples in L<perlfaq4> have been updated to show the use of
L<Time::Piece>.

=head1 Diagnostics

The following additions or changes have been made to diagnostic output,
including warnings and fatal error messages.  For the complete list of
diagnostic messages, see L<perldiag>.

=head2 New Diagnostics

=head3 New Errors

=over

=item Closure prototype called

This error occurs when a subroutine reference passed to an attribute
handler is called, if the subroutine is a closure [perl #68560].

=item Insecure user-defined property %s

Perl detected tainted data when trying to compile a regular
expression that contains a call to a user-defined character property
function, i.e. C<\p{IsFoo}> or C<\p{InFoo}>.
See L<perlunicode/User-Defined Character Properties> and L<perlsec>.

=item panic: gp_free failed to free glob pointer - something is repeatedly re-creating entries

This new error is triggered if a destructor called on an object in a
typeglob that is being freed creates a new typeglob entry containing an
object with a destructor that creates a new entry containing an object....

=item Parsing code internal error (%s)

This new fatal error is produced when parsing
code supplied by an extension violates the
parser's API in a detectable way.

=item refcnt: fd %d%s

This new error only occurs if a internal consistency check fails when a
pipe is about to be closed.

=item Regexp modifier "/%c" may not appear twice

The regular expression pattern has one of the
mutually exclusive modifiers repeated.

=item Regexp modifiers "/%c" and "/%c" are mutually exclusive

The regular expression pattern has more than one of the mutually
exclusive modifiers.

=item Using !~ with %s doesn't make sense

This error occurs when C<!~> is used with C<s///r> or C<y///r>.

=back

=head3 New Warnings

=over

=item "\b{" is deprecated; use "\b\{" instead

=item "\B{" is deprecated; use "\B\{" instead

Use of an unescaped "{" immediately following a C<\b> or C<\B> is now
deprecated so as to reserve its use for Perl itself in a future release.

=item Operation "%s" returns its argument for ...

Performing an operation requiring Unicode semantics (such as case-folding)
on a Unicode surrogate or a non-Unicode character now triggers a warning:
'Operation "%s" returns its argument for ...'.

=item Use of qw(...) as parentheses is deprecated

See L</"Use of qw(...) as parentheses">, above, for details.

=back

=head2 Changes to Existing Diagnostics

=over 4

=item *

The "Variable $foo is not imported" warning that precedes a
C<strict 'vars'> error has now been assigned the "misc" category, so that
C<no warnings> will suppress it [perl #73712].

=item *

C<warn> and C<die> now produce 'Wide character' warnings when fed a
character outside the byte range if STDERR is a byte-sized handle.

=item *

The 'Layer does not match this perl' error message has been replaced with
these more helpful messages [perl #73754]:

=over 4

=item *

PerlIO layer function table size (%d) does not match size expected by this
perl (%d)

=item *

PerlIO layer instance size (%d) does not match size expected by this perl
(%d)

=back

=item *

The "Found = in conditional" warning that is emitted when a constant is
assigned to a variable in a condition is now withheld if the constant is
actually a subroutine or one generated by C<use constant>, since the value
of the constant may not be known at the time the program is written
[perl #77762].

=item *

Previously, if none of the C<gethostbyaddr>, C<gethostbyname> and
C<gethostent> functions were implemented on a given platform, they would
all die with the message 'Unsupported socket function "gethostent" called',
with analogous messages for C<getnet*> and C<getserv*>. This has been
corrected.

=item *

The warning message about unrecognized regular expression escapes passed
through has been changed to include any literal '{' following the
two-character escape.  E.g., "\q{" is now emitted instead of "\q".

=back

=head1 Utility Changes

=head3 L<perlbug>

=over 4

=item *

L<perlbug> now looks in the EMAIL environment variable for a return address
if the REPLY-TO and REPLYTO variables are empty.

=item *

L<perlbug> did not previously generate a From: header, potentially
resulting in dropped mail.  Now it does include that header.

=item *

The user's address is now used as the return-path.

Many systems these days don't have a valid Internet domain name and
perlbug@perl.org does not accept email with a return-path that does
not resolve.  So the user's address is now passed to sendmail so it's
less likely to get stuck in a mail queue somewhere [perl #82996].

=back

=head3 L<perl5db.pl>

=over

=item *

The remote terminal works after forking and spawns new sessions - one
for each forked process.

=back

=head3 L<ptargrep>

=over 4

=item *

L<ptargrep> is a new utility to apply pattern matching to the contents of
files  in a tar archive.  It comes with C<Archive::Tar>.

=back

=head1 Configuration and Compilation

See also L</"Naming fixes in Policy_sh.SH may invalidate Policy.sh">,
above.

=over 4

=item *

CCINCDIR and CCLIBDIR for the mingw64
cross-compiler are now correctly under
$(CCHOME)\mingw\include and \lib rather than immediately below $(CCHOME).

This means the 'incpath', 'libpth', 'ldflags', 'lddlflags' and
'ldflags_nolargefiles' values in Config.pm and Config_heavy.pl are now
set correctly.

=item *

'make test.valgrind' has been adjusted to account for cpan/dist/ext
separation.

=item *

On compilers that support it, C<-Wwrite-strings> is now added to cflags by
default.

=item *

The C<Encode> module can now (once again) be included in a static Perl
build.  The special-case handling for this situation got broken in Perl
5.11.0, and has now been repaired.

=item *

The previous default size of a PerlIO buffer (4096 bytes) has been increased
to the larger of 8192 bytes and your local BUFSIZ.  Benchmarks show that doubling
this decade-old default increases read and write performance in the neighborhood
of 25% to 50% when using the default layers of perlio on top of unix.  To choose
a non-default size, such as to get back the old value or to obtain an even
larger value, configure with:

     ./Configure -Accflags=-DPERLIOBUF_DEFAULT_BUFSIZ=N

where N is the desired size in bytes; it should probably be a multiple of
your page size.

=back

=head1 Testing

XXX This section has not been cleaned up yet. Do we really need this
section? How many people are going to find a list of new test files useful?
Anyone hacking on the core can use
‘git diff v5.12.0..v5.14.0 MANIFEST|grep '^[+-]t/'’.

XXX Any significant changes to the testing of a freshly built perl should be
listed here.  Changes which create B<new> files in F<t/> go here as do any
large changes to the testing harness (e.g. when parallel testing was added).
Changes to existing files in F<t/> aren't worth summarising, although the bugs
that they represent may be covered elsewhere.

=over 4

=item *

F<t/harness> clears PERL5LIB, PERLLIB, PERL5OPT as t/TEST does (a2d3de1)

=item *

Many common testing routines were refactored into t/lib/common.pl

=item *

Several test files have been modernized to use Test::More

=item *

F<t/op/print.t> has been added to test implicit printing of C<$_>.

=item *

F<t/io/errnosig.t> has been added to test for restoration of of C<$!> when
leaving signal handlers.

=item *

F<t/op/tie_fetch_count.t> has been added to see if C<FETCH> is only called once
on tied variables.

=item *

F<lib/Tie/ExtraHash.t> has been added to make sure the, previously untested,
L<Tie::ExtraHash> keeps working.

=item *

F<t/re/overload.t> has been added to test against string corruption in pattern
matches on overloaded objects. This is a TODO test.

=item *

The new F<t/lib/universal.t> script tests the Internal::* functions and other
things in F<universal.c>.

=item *

A rare race condition in F<t/op/while_readdir.t> has been fixed, stopping it
from failing randomly when running tests in parallel.

=item *

The new F<t/op/leaky-magic.t> script tests that magic applied to variables in
the main packages does not affect other packages.

=item *

The script F<t/op/threads-dirh.t> has been added, which tests interaction
of threads and directory handles.

=item *

The new F<t/mro/isa_aliases.t> has been added, which tests that
C<*Foo::ISA = *Bar::ISA> works properly.

=item *

F<t/mro/isarev.t> has been added, which tests that C<PL_isarev> (accessible
at the Perl level via C<mro::get_isarev>) is updated properly.

=item *

F<t/run/switchd-78586.t> has been added, which tests that [perl #78586]
has been fixed (related to line numbers in the debugger).

=item *

C<lib/File/DosGlob.t> has been modernized and now uses C<Test::More>.

=item *

A new test script, C<t/porting/filenames.t>, makes sure that filenames and
paths are reasonably portable.

=item *

C<t/porting/diag.t> is now several orders of magnitude faster.

=item *

C<t/porting/buildtoc.t> now tests that the documentation TOC file is current and well-formed.

=item *

C<t/base/while.t> now tests the basics of a while loop with minimal dependencies.

=item *

C<t/cmd/while.t> now uses F<test.pl> for better maintainability.

=item *

C<t/op/split.t> now tests calls to C<split> without any pattern specified.

=item *

F<porting/FindExt.t> now skips all tests on a static (-Uusedl) build
of perl.

=item *

F<porting/FindExt.t> now passes on non-Win32 platforms when some
extensions are built statically.

=item *

The tests for C<split /\s/> and Unicode have been moved from
F<t/op/split.t> to the new F<t/op/split_unicode.t>.

=item *

F<t/re/re.t> has been moved to F<ext/re/t/re_funcs_u.t>.

=item *

The tests for [perl #72922] have been moved from F<t/re/qr.t> to the new
F<t/re/qr-72922.t>.

=item *

F<t/re/reg_unsafe.t> has been deleted and its only test moved to
F<t/re/pat_advanced.t>.

=back

=head1 Platform Support

=head2 New Platforms

=over 4

=item AIX

Perl now builds on AIX 4.2.

=back

=head2 Discontinued Platforms

=over 4

=item Apollo DomainOS

The last vestiges of support for this platform have been excised from the
Perl distribution. It was officially discontinued in version 5.12.0. It had
not worked for years before that.

=item MacOS Classic

The last vestiges of support for this platform have been excised from the
Perl distribution. It was officially discontinued in an earlier version.

=back

=head2 Platform-Specific Notes

=head3 Cygwin

=over 4

=item *

MakeMaker has been updated to build man pages on cygwin.

=item *

Improved rebase behaviour

If a dll is updated on cygwin the old imagebase address is reused.
This solves most rebase errors, especially when updating on core dll's.
See L<http://www.tishler.net/jason/software/rebase/rebase-2.4.2.README> for more information.

=item *

Support for the standard cygwin dll prefix, which is e.g. needed for FFI's

=item *

Updated build hints file

=back

=head3 IRIX

Conversion of strings to floating-point numbers is now more accurate on
IRIX systems [perl #32380].

=head3 Mac OS X

Early versions of Mac OS X (Darwin) had buggy implementations of the
C<setregid>, C<setreuid>, C<setrgid> and C<setruid> functions, so perl
would pretend they did not exist.

These functions are now recognised on Mac OS 10.5 (Leopard; Darwin 9) and
higher, as they have been fixed [perl #72990].

=head3 MirBSD

Previously if you built perl with a shared libperl.so on MirBSD (the
default config), it would work up to the installation; however, once
installed, it would be unable to find libperl.  So path handling is now
treated as in the other BSD dialects.

=head3 NetBSD

The NetBSD hints file has been changed to make the system's malloc the
default.

=head3 Recent OpenBSDs now use perl's malloc

OpenBSD E<gt> 3.7 has a new malloc implementation which is mmap-based
and as such can release memory back to the OS; however, perl's use of
this malloc causes a substantial slowdown so we now default to using
perl's malloc instead [perl #75742].

=head3 OpenVOS

perl now builds again with OpenVOS (formerly known as Stratus VOS)
[perl #78132].

=head3 Solaris

DTrace is now supported on Solaris. There used to be build failures, but
these have been fixed [perl #73630].

=head3 VMS

=over

=item *

C<PerlIOUnix_open> now honours the default permissions on VMS.

When C<perlio> became the default and C<unixio> became the default bottom layer,
the most common path for creating files from Perl became C<PerlIOUnix_open>,
which has always explicitly used C<0666> as the permission mask.

To avoid this, C<0777> is now passed as the permissions to C<open()>.  In the
VMS CRTL, C<0777> has a special meaning over and above intersecting with the
current umask; specifically, it allows Unix syscalls to preserve native default
permissions.

=item *

The shortening of symbols longer than 31 characters in the C sources is
now done by the compiler rather than by xsubpp (which could only do so
for generated symbols in XS code).

=item *

Record-oriented files (record format variable or variable with fixed control)
opened for write by the perlio layer will now be line-buffered to prevent the
introduction of spurious line breaks whenever the perlio buffer fills up.

=back

=head3 Windows

See also L</"fork() emulation will not wait for signalled children"> and
L</"Perl source code is read in text mode on Windows">, above.

=over 4

=item *

Fixed build process for SDK2003SP1 compilers.

=item *

When using old 32-bit compilers, the define C<_USE_32BIT_TIME_T> will now
be set in C<$Config{ccflags}>.  This improves portability when compiling
XS extensions using new compilers, but for a perl compiled with old 32-bit
compilers.

=item *

C<$Config{gccversion}> is now set correctly when perl is built using the
mingw64 compiler from L<http://mingw64.org> [perl #73754].

=item *

The build process proceeds more smoothly with mingw and dmake when
F<C:\MSYS\bin> is in the PATH, due to a C<Cwd> fix.

=item *

Support for building with Visual C++ 2010 is now underway, but is not yet
complete.  See F<README.win32> or L<perlwin32> for more details.

=item *

The option to use an externally-supplied C<crypt()>, or to build with no
C<crypt()> at all, has been removed.  Perl supplies its own C<crypt()>
implementation for Windows, and the political situation that required
this part of the distribution to sometimes be omitted is long gone.

=back

=head1 Internal Changes

=head2 New APIs

=head3 CLONE_PARAMS structure added to ease correct thread creation

Modules that create threads should now create C<CLONE_PARAMS> structures
by calling the new function C<Perl_clone_params_new()>, and free them with
C<Perl_clone_params_del()>.  This will ensure compatibility with any future
changes to the internals of the C<CLONE_PARAMS> structure layout, and that
it is correctly allocated and initialised.

=head3 New parsing functions

Several functions have been added for parsing statements or multiple
statements:

=over

=item *

C<parse_fullstmt> parses a complete Perl statement.

=item *

C<parse_stmtseq> parses a sequence of statements, up
to closing brace or EOF.

=item *

C<parse_block> parses a block [perl #78222].

=item *

C<parse_barestmt> parses a statement
without a label.

=item *

C<parse_label> parses a statement label, separate from statements.

=back

The
L<C<parse_fullexpr()>|perlapi/parse_fullexpr>,
L<C<parse_listexpr()>|perlapi/parse_listexpr>,
L<C<parse_termexpr()>|perlapi/parse_termexpr>, and
L<C<parse_arithexpr()>|perlapi/parse_arithexpr>
functions have been added to the API.  They perform
recursive-descent parsing of expressions at various precedence levels.
They are expected to be used by syntax plugins.

See L<perlapi> for details.

=head3 Hints hash API

A new C API for introspecting the hinthash C<%^H> at runtime has been
added.  See C<cop_hints_2hv>, C<cop_hints_fetchpvn>, C<cop_hints_fetchpvs>,
C<cop_hints_fetchsv>, and C<hv_copy_hints_hv> in L<perlapi> for details.

A new, experimental API has been added for accessing the internal
structure that Perl uses for C<%^H>.  See the functions beginning with
C<cophh_> in L<perlapi>.

=head3 C interface to C<caller()>

The C<caller_cx> function has been added as an XSUB-writer's equivalent of
C<caller()>.  See L<perlapi> for details.

=head3 Custom per-subroutine check hooks

XS code in an extension module can now annotate a subroutine (whether
implemented in XS or in Perl) so that nominated XS code will be called
at compile time (specifically as part of op checking) to change the op
tree of that subroutine.  The compile-time check function (supplied by
the extension module) can implement argument processing that can't be
expressed as a prototype, generate customised compile-time warnings,
perform constant folding for a pure function, inline a subroutine
consisting of sufficiently simple ops, replace the whole call with a
custom op, and so on.  This was previously all possible by hooking the
C<entersub> op checker, but the new mechanism makes it easy to tie the
hook to a specific subroutine.  See L<perlapi/cv_set_call_checker>.

To help in writing custom check hooks, several subtasks within standard
C<entersub> op checking have been separated out and exposed in the API.

=head3 Improved support for custom OPs

Custom ops can now be registered with the new C<custom_op_register> C
function and the C<XOP> structure.  This will make it easier to add new
properties of custom ops in the future.  Two new properties have been added
already, C<xop_class> and C<xop_peep>.

C<xop_class> is one of the OA_*OP constants, and allows L<B> and other
introspection mechanisms to work with custom ops
that aren't BASEOPs.  C<xop_peep> is a pointer to
a function that will be called for ops of this
type from C<Perl_rpeep>.

See L<perlguts/Custom Operators> and L<perlapi/Custom Operators> for more
detail.

The old C<PL_custom_op_names>/C<PL_custom_op_descs> interface is still
supported but discouraged.

=head3 Scope hooks

It is now possible for XS code to hook into Perl's lexical scope
mechanism at compile time, using the new C<Perl_blockhook_register>
function. See L<perlguts/"Compile-time scope hooks">.

=head3 The recursive part of the peephole optimizer is now hookable

In addition to C<PL_peepp>, for hooking into the toplevel peephole optimizer, a
C<PL_rpeepp> is now available to hook into the optimizer recursing into
side-chains of the optree.

=head3 New non-magical variants of existing functions

The following functions/macros have been added to the API.  The C<*_nomg>
macros are equivalent to their non-_nomg variants, except that they ignore
get-magic.  Those ending in C<_flags> allow one to specify whether
get-magic is processed.

  sv_2bool_flags
  SvTRUE_nomg
  sv_2nv_flags
  SvNV_nomg
  sv_cmp_flags
  sv_cmp_locale_flags
  sv_eq_flags
  sv_collxfrm_flags

In some of these cases, the non-_flags functions have
been replaced with wrappers around the new functions. 

=head3 pv/pvs/sv versions of existing functions

Many functions ending with pvn now have equivalent pv/pvs/sv versions.

=head3 List op-building functions

List op-building functions have been added to the
API.  See L<op_append_elem|perlapi/op_append_elem>,
L<op_append_list|perlapi/op_append_list>, and
L<op_prepend_elem|perlapi/op_prepend_elem> in L<perlapi>.

=head3 C<LINKLIST>

The L<LINKLIST|perlapi/LINKLIST> macro, part of op building that
constructs the execution-order op chain, has been added to the API.

=head3 Localisation functions

The C<save_freeop>, C<save_op>, C<save_pushi32ptr> and C<save_pushptrptr>
functions have been added to the API.

=head3 Stash names

A stash can now have a list of effective names in addition to its usual
name.  The first effective name can be accessed via the C<HvENAME> macro,
which is now the recommended name to use in MRO linearisations (C<HvNAME>
being a fallback if there is no C<HvENAME>).

These names are added and deleted via C<hv_ename_add> and
C<hv_ename_delete>.  These two functions are I<not> part of the API.

=head3 New functions for finding and removing magic

The L<C<mg_findext()>|perlapi/mg_findext> and
L<C<sv_unmagicext()>|perlapi/sv_unmagicext>
functions have been added to the API.
They allow extension authors to find and remove magic attached to
scalars based on both the magic type and the magic virtual table, similar to how
C<sv_magicext()> attaches magic of a certain type and with a given virtual table
to a scalar. This eliminates the need for extensions to walk the list of
C<MAGIC> pointers of an C<SV> to find the magic that belongs to them.

=head3 C<find_rundefsv>

This function returns the SV representing C<$_>, whether it's lexical
or dynamic.

=head3 C<Perl_croak_no_modify>

C<Perl_croak_no_modify()> is short-hand for
C<Perl_croak("%s", PL_no_modify)>.

=head3 C<PERL_STATIC_INLINE> define

The C<PERL_STATIC_INLINE> define has been added to provide the best-guess
incantation to use for static inline functions, if the C compiler supports
C99-style static inline.  If it doesn't, it'll give a plain C<static>.

C<HAS_STATIC_INLINE> can be used to check if the compiler actually supports
inline functions.

=head3 New C<pv_escape> option for hexadecimal escapes

A new option, C<PERL_PV_ESCAPE_NONASCII>, has been added to C<pv_escape> to
dump all characters above ASCII in hexadecimal.  Before, one could get all
characters as hexadecimal or the Latin1 non-ASCII as octal.

=head3 C<lex_start>

C<lex_start> has been added to the API, but is considered experimental.

=head3 C<op_scope()> and C<op_lvalue()>

The C<op_scope()> and C<op_lvalue()> functions have been added to the API,
but are considered experimental.

=head2 C API Changes

=head3 C<PERL_POLLUTE> has been removed

The option to define C<PERL_POLLUTE> to expose older 5.005 symbols for
backwards compatibility has been removed.  It's use was always discouraged,
and MakeMaker contains a more specific escape hatch:

    perl Makefile.PL POLLUTE=1

This can be used for modules that have not been upgraded to 5.6 naming
conventions (and really should be completely obsolete by now).

=head3 Check API compatibility when loading XS modules

When perl's API changes in incompatible ways (which usually happens between
major releases), XS modules compiled for previous versions of perl will not
work anymore.  They will need to be recompiled against the new perl.

In order to ensure that modules are recompiled, and to prevent users from
accidentally loading modules compiled for old perls into newer ones, the
C<XS_APIVERSION_BOOTCHECK> macro has been added.  That macro, which is
called when loading every newly compiled extension, compares the API
version of the running perl with the version a module has been compiled for
and raises an exception if they don't match.

=head3 Perl_fetch_cop_label

The first argument of the C API function C<Perl_fetch_cop_label> has changed
from C<struct refcounted he *> to C<COP *>, to insulate the user from
implementation details.

This API function was marked as "may change", and likely isn't in use outside
the core.  (Neither an unpacked CPAN, nor Google's codesearch, finds any other
references to it.)

=head3 GvCV() and GvGP() are no longer lvalues

The new GvCV_set() and GvGP_set() macros are now provided to replace
assignment to those two macros.

This allows a future commit to eliminate some backref magic between GV
and CVs, which will require complete control over assignment to the
gp_cv slot.

=head3 CvGV() is no longer an lvalue

Under some circumstances, the C<CvGV()> field of a CV is now
reference-counted.  To ensure consistent behaviour, direct assignment to
it, for example C<CvGV(cv) = gv> is now a compile-time error.  A new macro,
C<CvGV_set(cv,gv)> has been introduced to perform this operation
safely.  Note that modification of this field is not part of the public
API, regardless of this new macro (and despite its being listed in this section).

=head3 CvSTASH() is no longer an lvalue

The C<CvSTASH()> macro can now only be used as an rvalue.  C<CvSTASH_set()>
has been added to replace assignment to C<CvSTASH()>.  This is to ensure
that backreferences are handled properly.  These macros are not part of the
API.

=head3 Calling conventions for C<newFOROP> and C<newWHILEOP>

The way the parser handles labels has been cleaned up and refactored. As a
result, the C<newFOROP()> constructor function no longer takes a parameter
stating what label is to go in the state op.

The C<newWHILEOP()> and C<newFOROP()> functions no longer accept a line
number as a parameter.

=head3 Flags passed to C<uvuni_to_utf8_flags> and C<utf8n_to_uvuni>

Some of the flags parameters to uvuni_to_utf8_flags() and
utf8n_to_uvuni() have changed.  This is a result of Perl's now allowing
internal storage and manipulation of code points that are problematic
in some situations.  Hence, the default actions for these functions has
been complemented to allow these code points.  The new flags are
documented in L<perlapi>.  Code that requires the problematic code
points to be rejected needs to change to use these flags.  Some flag
names are retained for backward source compatibility, though they do
nothing, as they are now the default.  However the flags
C<UNICODE_ALLOW_FDD0>, C<UNICODE_ALLOW_FFFF>, C<UNICODE_ILLEGAL>, and
C<UNICODE_IS_ILLEGAL> have been removed, as they stem from a
fundamentally broken model of how the Unicode non-character code points
should be handled, which is now described in
L<perlunicode/Non-character code points>.  See also L</Selected Bug Fixes>.

XXX Which bugs in particular? Selected Bug Fixes is too long for this link
to be meaningful right now.

=head2 Deprecated C APIs

=over

=item C<Perl_ptr_table_clear>

C<Perl_ptr_table_clear> 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.

=item C<sv_compile_2op>

The C<sv_compile_2op()> API function is now deprecated.  Searches suggest
that nothing on CPAN is using it, so this should have zero impact.

It attempted to provide an API to compile code down to an optree, but failed
to bind correctly to lexicals in the enclosing scope.  It's not possible to
fix this problem within the constraints of its parameters and return value.

=item C<find_rundefsvoffset>

The C<find_rundefsvoffset> function has been deprecated.  It appeared that
its design was insufficient for reliably getting the lexical C<$_> at
run-time.

Use the new C<find_rundefsv> function or the C<UNDERBAR> macro
instead. They directly return the right SV representing C<$_>, whether it's
lexical or dynamic.

=item C<CALL_FPTR> and C<CPERLscope>

Those are left from an old implementation of C<MULTIPLICITY> using C++ objects,
which was removed in Perl 5.8.  Nowadays these macros do exactly nothing, so
they shouldn't be used anymore.

For compatibility, they are still defined for external C<XS> code.  Only
extensions defining C<PERL_CORE> must be updated now.

=back

=head2 Other Internal Changes

=head3 Stack unwinding

The protocol for unwinding the C stack at the last stage of a C<die>
has changed how it identifies the target stack frame.  This now uses
a separate variable C<PL_restartjmpenv>, where previously it relied on
the C<blk_eval.cur_top_env> pointer in the C<eval> context frame that
has nominally just been discarded.  This change means that code running
during various stages of Perl-level unwinding no longer needs to take
care to avoid destroying the ghost frame.

=head3 Scope stack entries

The format of entries on the scope stack has been changed, resulting in a
reduction of memory usage of about 10%.  In particular, the memory used by
the scope stack to record each active lexical variable has been halved.

=head3 Memory allocation for pointer tables

Memory allocation for pointer tables has been changed.  Previously
C<Perl_ptr_table_store> allocated memory from the same arena system as
C<SV> bodies and C<HE>s, with freed memory remaining bound to those arenas
until interpreter exit.  Now it allocates memory from arenas private to the
specific pointer table, and that memory is returned to the system when
C<Perl_ptr_table_free> is called.  Additionally, allocation and release are
both less CPU intensive.

=head3 C<UNDERBAR>

The C<UNDERBAR> macro now calls C<find_rundefsv>.  C<dUNDERBAR> is now a
noop but should still be used to ensure past and future compatibility.

=head3 String comparison routines renamed

The ibcmp_* functions have been renamed and are now called foldEQ,
foldEQ_locale and foldEQ_utf8.  The old names are still available as
macros.

=head3 C<chop> and C<chomp> implementations merged

The opcode bodies for C<chop> and C<chomp> and for C<schop> and C<schomp>
have been merged.  The implementation functions C<Perl_do_chop()> and
C<Perl_do_chomp()>, never part of the public API, have been merged and
moved to a static function in F<pp.c>.  This shrinks the perl binary
slightly, and should not affect any code outside the core (unless it is
relying on the order of side effects when C<chomp> is passed a I<list> of
values).

=head1 Selected Bug Fixes

=head2 I/O

=over 4

=item *

Perl no longer produces this warning:

    $ perl -we 'open my $f, ">", \my $x; binmode $f, "scalar"'
    Use of uninitialized value in binmode at -e line 1.

=item *

Opening a glob reference via C<< open $fh, "E<gt>", \*glob >> will no longer
cause the glob to be corrupted when the filehandle is printed to.  This would
cause perl to crash whenever the glob's contents were accessed
[perl #77492].

=item *

PerlIO no longer crashes when called recursively, e.g., from a signal
handler. Now it just leaks memory [perl #75556].

=item *

Most I/O functions were not warning for unopened handles unless the
'closed' and 'unopened' warnings categories were both enabled. Now only
C<use warnings 'unopened'> is necessary to trigger these warnings (as was
always meant to be the case).

=item *

There have been several fixes to PerlIO layers:

When C<binmode FH, ":crlf"> pushes the C<:crlf> layer on top of the stack,
it no longer enables crlf layers lower in the stack, to avoid unexpected
results [perl #38456].

Opening a file in C<:raw> mode now does what it advertises to do (first
open the file, then binmode it), instead of simply leaving off the top
layer [perl #80764].

The three layers C<:pop>, C<:utf8> and C<:bytes> didn't allow stacking when
opening a file.  For example
this:

    open FH, '>:pop:perlio', 'some.file' or die $!;

Would throw an error: "Invalid argument".  This has been fixed in this
release [perl #82484].

=back

=head2 Regular Expression Bug Fixes

=over

=item *

The regular expression engine no longer loops when matching
C<"\N{LATIN SMALL LIGATURE FF}" =~ /f+/i> and similar expressions
[perl #72998].

=item *

The trie runtime code should no longer allocate massive amounts of memory,
fixing #74484.

=item *

Syntax errors in C<< (?{...}) >> blocks no longer cause panic messages
[perl #2353].

=item *

A pattern like C<(?:(o){2})?> no longer causes a "panic" error
[perl #39233].

=item *

A fatal error in regular expressions containing C<(.*?)> when processing
UTF-8 data has been fixed [perl #75680].

=item *

An erroneous regular expression engine optimisation that caused regex verbs like
C<*COMMIT> sometimes to be ignored has been removed.

=item *

The regular expression bracketed character class C<[\8\9]> was effectively the
same as C<[89\000]>, incorrectly matching a NULL character.  It also gave
incorrect warnings that the C<8> and C<9> were ignored.  Now C<[\8\9]> is the
same as C<[89]> and gives legitimate warnings that C<\8> and C<\9> are
unrecognized escape sequences, passed-through.

=item *

A regular expression match in the right-hand side of a global substitution
(C<s///g>) that is in the same scope will no longer cause match variables
to have the wrong values on subsequent iterations. This can happen when an
array or hash subscript is interpolated in the right-hand side, as in
C<s|(.)|@a{ print($1), /./ }|g> [perl #19078].

=item *

Several cases in which characters in the Latin-1 non-ASCII range (0x80 to
0xFF) used not to match themselves or used to match both a character class
and its complement have been fixed.  For instance, U+00E2 could match both
C<\w> and C<\W> [perl #78464] [perl #18281] [perl #60156].

=item *

Matching a Unicode character against an alternation containing characters
that happened to match continuation bytes in the former's UTF8
representation (C<qq{\x{30ab}} =~ /\xab|\xa9/>) would cause erroneous
warnings [perl #70998].

=item *

The trie optimisation was not taking empty groups into account, preventing
'foo' from matching C</\A(?:(?:)foo|bar|zot)\z/> [perl #78356].

=item *

A pattern containing a C<+> inside a lookahead would sometimes cause an
incorrect match failure in a global match (e.g., C</(?=(\S+))/g>)
[perl #68564].

=item *

A regular expression optimisation would sometimes cause a match with a
C<{n,m}> quantifier to fail when it should match [perl #79152].

=item *

Case insensitive matching in regular expressions compiled under C<use
locale> now works much more sanely when the pattern or
target string is encoded internally in
UTF8.  Previously, under these conditions the localeness
was completely lost.  Now, code points above 255 are treated as Unicode,
but code points between 0 and 255 are treated using the current locale
rules, regardless of whether the pattern or the string is encoded in UTF8.
The few case-insensitive matches that cross the 255/256 boundary are not
allowed.  For example, 0xFF does not caselessly match the character at
0x178, LATIN CAPITAL LETTER Y WITH DIAERESIS, because 0xFF may not be
LATIN SMALL LETTER Y in the current locale, and Perl has no way of
knowing if that character even exists in the locale, much less what code
point it is.

=item *

The C<(?|...)> regular expression construct no longer crashes if the final
branch has more sets of capturing parentheses than any other branch. This
was fixed in Perl 5.10.1 for the case of a single branch, but that fix did
not take multiple branches into account [perl #84746].

=item *

A bug has been fixed in the implementation of C<{...}> quantifiers in
regular expressions that prevented the code block in
C</((\w+)(?{ print $2 })){2}/> from seeing the C<$2> sometimes
[perl #84294].

=back

=head2 Syntax/Parsing Bugs

=over

=item *

C<when(scalar){...}> no longer crashes, but produces a syntax error
[perl #74114].

=item *

A label right before a string eval (C<foo: eval $string>) no longer causes
the label to be associated also with the first statement inside the eval
[perl #74290] (5.12.1).

=item *

The C<no 5.13.2;> form of C<no> no longer tries to turn on features or
pragmata (i.e., strict) [perl #70075].

=item *

C<BEGIN {require 5.12.0}> now behaves as documented, rather than behaving
identically to C<use 5.12.0;>.  Previously, C<require> in a C<BEGIN> block
was erroneously executing the C<use feature ':5.12.0'> and
C<use strict;> behaviour, which only C<use> was documented to
provide [perl #69050].

=item *

Lvalue subroutines are again able to return copy-on-write scalars.  This
had been broken since version 5.10.0 [perl #75656].

=item *

A regression introduced in Perl 5.12.0, making
C<< my $x = 3; $x = length(undef) >> result in C<$x> set to C<3> has been
fixed.  C<$x> will now be C<undef> [perl #85508].

=item *

When strict 'refs' mode is off, C<%{...}> in rvalue context returns
C<undef> if its argument is undefined.  An optimisation introduced in perl
5.12.0 to make C<keys %{...}> faster when used as a boolean did not take
this into account, causing C<keys %{+undef}> (and C<keys %$foo> when
C<$foo> is undefined) to be an error, which it should only be in strict
mode [perl #81750].

=item *

Constant-folding used to cause

  $text =~ ( 1 ? /phoo/ : /bear/)

to turn into

  $text =~ /phoo/

at compile time. Now it correctly matches against C<$_> [perl #20444].

=item *

Parsing Perl code (either with string C<eval> or by loading modules) from
within a C<UNITCHECK> block no longer causes the interpreter to crash
[perl #70614].

=item *

String evals no longer fail after 2 billion scopes have been
compiled [perl #83364].

=item *

The parser no longer hangs when encountering certain Unicode characters,
such as U+387 [perl #74022].

=item *

Several contexts no longer allow a Unicode character to begin a word
that should never begin words, for an example an accent that must follow
another character previously could precede all other characters.

=item *

Defining a constant with the same name as one of perl's special blocks
(e.g., INIT) stopped working in 5.12.0, but has now been fixed
[perl #78634].

=item *

A reference to a literal value used as a hash key (C<$hash{\"foo"}>) used
to be stringified, even if the hash was tied [perl #79178].

=item *

A closure containing an C<if> statement followed by a constant or variable
is no longer treated as a constant [perl #63540].

=item *

C<state> can now be used with attributes. It used to mean the same thing as
C<my> if attributes were present [perl #68658].

=item *

Expressions like C<< @$a > 3 >> no longer cause C<$a> to be mentioned in
the "Use of uninitialized value in numeric gt" warning when C<$a> is
undefined (since it is not part of the C<E<gt>> expression, but the operand
of the C<@>) [perl #72090].

=item *

C<require> no longer causes C<caller> to return the wrong file name for
the scope that called C<require> and other scopes higher up that had the
same file name [perl #68712].

=item *

Accessing an element of a package array with a hard-coded number (as
opposed to an arbitrary expression) would crash if the array did not exist.
Usually the array would be autovivified during compilation, but typeglob
manipulation could remove it, as in these two cases which used to crash:

  *d = *a;  print $d[0];
  undef *d; print $d[0];

=back

=head2 Stashes, Globs and Method Lookup

Perl 5.10.0 introduced a new internal mechanism for caching MROs (method
resolution orders, or lists of parent classes; aka "isa" caches) to make
method lookup faster (so @ISA arrays would not have to be searched
repeatedly).  Unfortunately, this brought with it quite a few bugs.  Almost
all of these have been fixed now, along with a few MRO-related bugs that
existed before 5.10.0:

=over

=item *

The following used to have erratic effects on method resolution, because
the "isa" caches were not reset or otherwise ended up listing the wrong
classes.  These have been fixed.

=over

=item Aliasing packages by assigning to globs [perl #77358]

=item Deleting packages by deleting their containing stash elements

=item Undefining the glob containing a package (C<undef *Foo::>)

=item Undefining an ISA glob (C<undef *Foo::ISA>)

=item Deleting an ISA stash element (C<delete $Foo::{ISA}>)

=item Sharing @ISA arrays between classes (via C<*Foo::ISA = \@Bar::ISA> or
C<*Foo::ISA = *Bar::ISA>) [perl #77238]

=back

C<undef *Foo::ISA> would even stop a new C<@Foo::ISA> array from updating
caches.

=item *

Typeglob assignments would crash if the glob's stash no longer existed, so
long as the glob assigned to was named 'ISA' or the glob on either side of
the assignment contained a subroutine.

=item *

C<PL_isarev>, which is accessible to Perl via C<mro::get_isarev> is now
updated properly when packages are deleted or removed from the C<@ISA> of
other classes. This allows many packages to be created and deleted without
causing a memory leak [perl #75176].

=back

In addition, various other bugs related to typeglobs and stashes have been
fixed:

=over 

=item *

Some work has been done on the internal pointers that link between symbol
tables (stashes), typeglobs and subroutines.  This has the effect that
various edge cases related to deleting stashes or stash entries (e.g.
<%FOO:: = ()>), and complex typeglob or code reference aliasing, will no
longer crash the interpreter.

=item *

Assigning a reference to a glob copy now assigns to a glob slot instead of
overwriting the glob with a scalar [perl #1804] [perl #77508].

=item *

A bug when replacing the glob of a loop variable within the loop has been fixed
[perl #21469].  This
means the following code will no longer crash:

    for $x (...) {
        *x = *y;
    }

=item *

Assigning a glob to a PVLV used to convert it to a plain string. Now it
works correctly, and a PVLV can hold a glob. This would happen when a
nonexistent hash or array element was passed to a subroutine:

  sub { $_[0] = *foo }->($hash{key});
  # $_[0] would have been the string "*main::foo"

It also happened when a glob was assigned to, or returned from, an element
of a tied array or hash [perl #36051].

=item *

When trying to report C<Use of uninitialized value $Foo::BAR>, crashes could
occur if the glob holding the global variable in question had been detached
from its original stash by, for example, C<delete $::{'Foo::'}>.  This has
been fixed by disabling the reporting of variable names in those
cases.

=item *

During the restoration of a localised typeglob on scope exit, any
destructors called as a result would be able to see the typeglob in an
inconsistent state, containing freed entries, which could result in a
crash. This would affect code like this:

  local *@;
  eval { die bless [] }; # puts an object in $@
  sub DESTROY {
    local $@; # boom
  }

Now the glob entries are cleared before any destructors are called. This
also means that destructors can vivify entries in the glob. So perl tries
again and, if the entries are re-created too many times, dies with a
'panic: gp_free...' error message.

=back

=head2 Unicode

=over

=item *

What has become known as the "Unicode Bug" is mostly resolved in this release.
Under C<use feature 'unicode_strings'>, the internal storage format of a
string no longer affects the external semantics.  There are two known
exceptions.  User-defined case changing functions, which are planned to
be deprecated in 5.14, require utf8-encoded strings to function; and the
character C<LATIN SMALL LETTER SHARP S> in regular expression
case-insensitive matching has a somewhat different set of bugs depending
on the internal storage format.  Case-insensitive matching of all
characters that have multi-character matches, as this one does, is
problematical in Perl [perl #58182].

=item *

The handling of Unicode non-characters has changed.
Previously they were mostly considered illegal, except that only one of
the 66 of them was known about in places.  The Unicode standard
considers them legal, but forbids the "open interchange" of them.
This is part of the change to allow the internal use of any code point
(see L</Core Enhancements>).  Together, these changes resolve
[perl #38722], [perl #51918], [perl #51936], [perl #63446].

=item *

Naming a deprecated character in \N{...} no longer leaks memory.

=item *

C<chop> now correctly handles characters above "\x{7fffffff}"
[perl #73246].

=item *

Passing to C<index> an offset beyond the end of the string when the string
is encoded internally in UTF8 no longer causes panics [perl #75898].

=item *

C<warn()> and C<die()> now respect utf8-encoded scalars [perl #45549].

=item *

Sometimes the UTF8 length cache would not be reset on a value
returned by substr, causing C<length(substr($uni_string,...))> to give
wrong answers.  With C<${^UTF8CACHE}> set to -1, it would produce a 'panic'
error message, too [perl #77692].

=back

=head2 Ties, Overloading and Other Magic

=over

=item *

Overloading now works properly in conjunction with tied
variables.  What formerly happened was that most ops checked their
arguments for overloading I<before> checking for magic, so for example
an overloaded object returned by a tied array access would usually be
treated as not overloaded [RT #57012].

=item *

Various cases of magic (e.g., tie methods) being called on tied variables
too many or too few times have been fixed:

=over

=item *

FETCH is no longer called on tied variables in void context.

=item *

C<$tied-E<gt>()> did not always call FETCH [perl #8438].

=item *

Filetest operators and C<y///> and C<tr///> were calling FETCH too
many times.

=item *

The C<=> operator used to ignore magic on its right-hand side if the
scalar happened to hold a typeglob (if a typeglob was the last thing
returned from or assigned to a tied scalar) [perl #77498].

=item *

Dereference operators used to ignore magic if the argument was a
reference already (e.g., from a previous FETCH) [perl #72144].

=item *

C<splice> now calls set-magic (so changes made
by C<splice @ISA> are respected by method calls) [perl #78400].

=back

=item *

String C<eval> now detects taintedness of overloaded or tied
arguments [perl #75716].

=item *

String C<eval> and regular expression matches against objects with string
overloading no longer cause memory corruption or crashes [perl 77084].

=item *

L<readline|perlfunc/"readline EXPR"> now honors C<< <> >> overloading on tied
arguments.

=item *

C<< E<lt>exprE<gt> >> always respects overloading now if the expression is
overloaded.

Due to the way that 'E<lt>E<gt> as glob' was parsed differently from
'E<lt>E<gt> as filehandle' from 5.6 onwards, something like C<< E<lt>$foo[0]E<gt> >> did
not handle overloading, even if C<$foo[0]> was an overloaded object.  This
was contrary to the documentation for overload, and meant that C<< E<lt>E<gt> >>
could not be used as a general overloaded iterator operator.

=item *

The fallback behaviour of overloading on binary operators was asymmetric
[perl #71286].

=item *

Magic applied to variables in the main package no longer affects other packages.
See L</Magic variables outside the main package> above [perl #76138].

=item *

Sometimes magic (ties, taintedness, etc.) attached to variables could cause
an object to last longer than it should, or cause a crash if a tied
variable were freed from within a tie method.  These have been fixed
[perl #81230].

=item *

DESTROY methods of objects implementing ties are no longer able to crash by
accessing the tied variable through a weak reference [perl #86328].

=back

=head2 The Debugger

=over

=item *

The Perl debugger now also works in taint mode [perl #76872].

=item *

Subroutine redefinition works once more in the debugger [perl #48332].

=item *

When C<-d> is used on the shebang (C<#!>) line, the debugger now has access
to the lines of the main program.  In the past, this sometimes worked and
sometimes did not, depending on what order things happened to be arranged
in memory [perl #71806].

=item *

A possible memory leak when using L<caller()|perlfunc/"caller EXPR"> to set
C<@DB::args> has been fixed.

=item *

Perl no longer stomps on $DB::single, $DB::trace and $DB::signal if they
already have values when $^P is assigned to [perl #72422].

=item *

C<#line> directives in string evals were not properly updating the arrays
of lines of code (C<< @{"_<..."} >>) that the debugger (or any debugging or
profiling module) uses. In threaded builds, they were not being updated at
all. In non-threaded builds, the line number was ignored, so any change to
the existing line number would cause the lines to be misnumbered
[perl #79442].

=back

=head2 Threads, Signals

=over

=item *

Don't accidentally clone lexicals in scope within active stack frames in
the parent when creating a child thread (RT #73086) (05d04d).

=item *

Defer signal handling when shared SV locks are held to avoid
deadlocks (RT #74868) (65c742).

=item *

Within signal handlers, C<$!> is now implicitly localized.

=item *

Several memory leaks in cloning and freeing threaded Perl interpreters have been
fixed [perl #77352].

=item *

Creating a new thread when directory handles were open used to cause a
crash, because the handles were not cloned, but simply passed to the new
thread, resulting in a double free.

Now directory handles are cloned properly, on systems that have a C<fchdir>
function. On other systems, new threads simply do not inherit directory
handles from their parent threads [perl #75154].

=item *

The typeglob C<*,>, which holds the scalar variable C<$,> (output field
separator), had the wrong reference count in child threads.

=item *

[perl #78494] When pipes are shared between threads, the C<close> function
(and any implicit close, such as on thread exit) no longer blocks.