[perl5.git] / pod / perldelta.pod

=encoding utf8

=head1 NAME

perldelta - what is new for perl v5.22.0

=head1 DESCRIPTION

This document describes differences between the 5.20.0 release and the 5.22.0
release.

If you are upgrading from an earlier release such as 5.18.0, first read
L<perl5200delta>, which describes differences between 5.18.0 and 5.20.0.

=head1 Core Enhancements

=head2 New bitwise operators

A new experimental facility has been added that makes the four standard
bitwise operators (C<& | ^ ~>) treat their operands consistently as
numbers, and introduces four new dotted operators (C<&. |. ^. ~.>) that
treat their operands consistently as strings.  The same applies to the
assignment variants (C<&= |= ^= &.= |.= ^.=>).

To use this, enable the "bitwise" feature and disable the
"experimental::bitwise" warnings category.  See L<perlop/Bitwise String
Operators> for details.
L<[perl #123466]|https://rt.perl.org/Ticket/Display.html?id=123466>.

=head2 New double-diamond operator

C<<< <<>> >>> is like C<< <> >> but uses three-argument C<open> to open
each file in C<@ARGV>.  This means that each element of C<@ARGV> will be treated
as an actual file name, and C<"|foo"> won't be treated as a pipe open.

=head2 New \b boundaries in regular expressions

=head3 qr/\b{gcb}/

C<gcb> stands for Grapheme Cluster Boundary.  It is a Unicode property
that finds the boundary between sequences of characters that look like a
single character to a native speaker of a language.  Perl has long had
the ability to deal with these through the C<\X> regular escape
sequence.  Now, there is an alternative way of handling these.  See
L<perlrebackslash/\b{}, \b, \B{}, \B> for details.

=head3 qr/\b{wb}/

C<wb> stands for Word Boundary.  It is a Unicode property
that finds the boundary between words.  This is similar to the plain
C<\b> (without braces) but is more suitable for natural language
processing.  It knows, for example, that apostrophes can occur in the
middle of words.  See L<perlrebackslash/\b{}, \b, \B{}, \B> for details.

=head3 qr/\b{sb}/

C<sb> stands for Sentence Boundary.  It is a Unicode property
to aid in parsing natural language sentences.
See L<perlrebackslash/\b{}, \b, \B{}, \B> for details.

=head2 C<no re> covers more and is lexical

Previously running C<no re> would only turn off a few things. Now it
turns off all the enabled things. For example, previously, you
couldn't turn off debugging, once enabled, inside the same block.

=head2 Non-Capturing Regular Expression Flag

Regular expressions now support a C</n> flag that disables capturing
and filling in C<$1>, C<$2>, etc... inside of groups:

  "hello" =~ /(hi|hello)/n; # $1 is not set

This is equivalent to putting C<?:> at the beginning of every capturing group.

See L<perlre/"n"> for more information.

=head2 C<use re 'strict'>

This applies stricter syntax rules to regular expression patterns
compiled within its scope, which hopefully will alert you to typos and
other unintentional behavior that backwards-compatibility issues prevent
us from doing in normal regular expression compilations.  Because the
behavior of this is subject to change in future Perl releases as we gain
experience, using this pragma will raise a category
C<experimental::re_strict> warning.
See L<'strict' in re|re/'strict' mode>.

=head2 Unicode 7.0 (with correction) is now supported

For details on what is in this release, see
L<http://www.unicode.org/versions/Unicode7.0.0/>.
The version of Unicode 7.0 that comes with Perl includes
a correction dealing with glyph shaping in Arabic
(see L<http://www.unicode.org/errata/#current_errata>).


=head2 S<C<use locale>> can restrict which locale categories are affected

It is now possible to pass a parameter to S<C<use locale>> to specify
a subset of locale categories to be locale-aware, with the remaining
ones unaffected.  See L<perllocale/The "use locale" pragma> for details.

=head2 Perl now supports POSIX 2008 locale currency additions

On platforms that are able to handle POSIX.1-2008, the
hash returned by
L<C<POSIX::localeconv()>|perllocale/The localeconv function>
includes the international currency fields added by that version of the
POSIX standard.  These are
C<int_n_cs_precedes>,
C<int_n_sep_by_space>,
C<int_n_sign_posn>,
C<int_p_cs_precedes>,
C<int_p_sep_by_space>,
and
C<int_p_sign_posn>.

=head2 Better heuristics on older platforms for determining locale UTF8ness

On platforms that implement neither the C99 standard nor the POSIX 2001
standard, determining if the current locale is UTF8 or not depends on
heuristics.  These are improved in this release.

=head2 Aliasing via reference

Variables and subroutines can now be aliased by assigning to a reference:

    \$c = \$d;
    \&x = \&y;

Or by using a backslash before a C<foreach> iterator variable, which is
perhaps the most useful idiom this feature provides:

    foreach \%hash (@array_of_hash_refs) { ... }

This feature is experimental and must be enabled via C<use feature
'refaliasing'>.  It will warn unless the C<experimental::refaliasing>
warnings category is disabled.

See L<perlref/Assigning to References>

=head2 C<prototype> with no arguments

C<prototype()> with no arguments now infers C<$_>.
L<[perl #123514]|https://rt.perl.org/Ticket/Display.html?id=123514>.

=head2 New "const" subroutine attribute

The "const" attribute can be applied to an anonymous subroutine.  It
causes the new sub to be executed immediately whenever one is created
(i.e. when the C<sub> expression is evaluated).  Its value is captured
and used to create a new constant subroutine that is returned.  This
feature is experimental.  See L<perlsub/Constant Functions>.

=head2 C<fileno> now works on directory handles

When the relevant support is available in the operating system, the
C<fileno> builtin now works on directory handles, yielding the
underlying file descriptor in the same way as for filehandles. On
operating systems without such support, C<fileno> on a directory handle
continues to return the undefined value, as before, but also sets C<$!> to
indicate that the operation is not supported.

Currently, this uses either a C<dd_fd> member in the OS C<DIR>
structure, or a C<dirfd(3)> function as specified by POSIX.1-2008.

=head2 List form of pipe open implemented for Win32

The list form of pipe:

  open my $fh, "-|", "program", @arguments;

is now implemented on Win32.  It has the same limitations as C<system
LIST> on Win32, since the Win32 API doesn't accept program arguments
as a list.

=head2 C<close> now sets C<$!>

When an I/O error occurs, the fact that there has been an error is recorded
in the handle.  C<close> returns false for such a handle.  Previously, the
value of C<$!> would be untouched by C<close>, so the common convention of
writing S<C<close $fh or die $!>> did not work reliably.  Now the handle
records the value of C<$!>, too, and C<close> restores it.

=head2 Assignment to list repetition

C<(...) x ...> can now be used within a list that is assigned to, as long
as the left-hand side is a valid lvalue.  This allows S<C<(undef,undef,$foo)
= that_function()>> to be written as S<C<((undef)x2, $foo) = that_function()>>.

=head2 Infinity and NaN (not-a-number) handling improved

Floating point values are able to hold the special values infinity (also
-infinity), and NaN (not-a-number).  Now we more robustly recognize and
propagate the value in computations, and on output normalize them to C<Inf> and
C<NaN>.

See also the L<POSIX> enhancements.

=head2 Floating point parsing has been improved

Parsing and printing of floating point values has been improved.

As a completely new feature, hexadecimal floating point literals
(like C<0x1.23p-4>)  are now supported, and they can be output with
C<printf %a>.

=head2 Packing infinity or not-a-number into a character is now fatal

Before, when trying to pack infinity or not-a-number into a
(signed) character, Perl would warn, and assumed you tried to
pack C<< 0xFF >>; if you gave it as an argument to C<< chr >>,
C<< U+FFFD >> was returned.

But now, all such actions (C<< pack >>, C<< chr >>, and C<< print '%c' >>)
result in a fatal error.

=head2 Experimental C Backtrace API

Perl now supports (via a C level API) retrieving
the C level backtrace (similar to what symbolic debuggers like gdb do).

The backtrace returns the stack trace of the C call frames,
with the symbol names (function names), the object names (like "perl"),
and if it can, also the source code locations (file:line).

The supported platforms are Linux and OS X (some *BSD might work at
least partly, but they have not yet been tested).

The feature needs to be enabled with C<Configure -Dusecbacktrace>.

See L<perlhacktips/"C backtrace"> for more information.

=head1 Security

=head2 Perl is now compiled with -fstack-protector-strong if available

Perl has been compiled with the anti-stack-smashing option
C<-fstack-protector> since 5.10.1.  Now Perl uses the newer variant
called C<-fstack-protector-strong>, if available.

=head2 The L<Safe> module could allow outside packages to be replaced

Critical bugfix: outside packages could be replaced.  L<Safe> has
been patched to 2.38 to address this.

=head2 Perl is now always compiled with -D_FORTIFY_SOURCE=2 if available

The 'code hardening' option called C<_FORTIFY_SOURCE>, available in
gcc 4.*, is now always used for compiling Perl, if available.

Note that this isn't necessarily a huge step since in many platforms
the step had already been taken several years ago: many Linux
distributions (like Fedora) have been using this option for Perl,
and OS X has enforced the same for many years.

=head1 Incompatible Changes

=head2 Subroutine signatures moved before attributes

The experimental sub signatures feature, as introduced in 5.20, parsed
signatures after attributes.  In this release, the positioning has been
moved such that signatures occur after the subroutine name (if any) and
before the attribute list (if any).

=head2 C<&> and C<\&> prototypes accepts only subs

The C<&> prototype character now accepts only anonymous subs (C<sub
{...}>), things beginning with C<\&>, or an explicit C<undef>.  Formerly
it erroneously also allowed references to arrays, hashes, and lists.
L<[perl #4539]|https://rt.perl.org/Ticket/Display.html?id=4539>.
L<[perl #123062]|https://rt.perl.org/Ticket/Display.html?id=123062>.
L<[perl #123062]|https://rt.perl.org/Ticket/Display.html?id=123475>.

In addition, the C<\&> prototype was allowing subroutine calls, whereas
now it only allows subroutines: C<&foo> is still permitted as an argument,
while C<&foo()> and C<foo()> no longer are.
L<[perl #77860]|https://rt.perl.org/Ticket/Display.html?id=77860>.

=head2 C<use encoding> is now lexical

The L<encoding> pragma's effect is now limited to lexical scope.  This
pragma is deprecated, but in the meantime, it could adversely affect
unrelated modules that are included in the same program.

=head2 List slices returning empty lists

List slices return an empty list now only if the original list was empty
(or if there are no indices).  Formerly, a list slice would return an empty
list if all indices fell outside the original list; now it returns a list
of undef values.
L<[perl #114498]|https://rt.perl.org/Ticket/Display.html?id=114498>.

=head2 C<\N{}> with a sequence of multiple spaces is now a fatal error

E.g. C<\N{TOO  MANY SPACES}> or C<\N{TRAILING SPACE }>.
This has been deprecated since v5.18.

=head2 S<C<use UNIVERSAL '...'>> is now a fatal error

Importing functions from C<UNIVERSAL> has been deprecated since v5.12, and
is now a fatal error.  S<C<"use UNIVERSAL">> without any arguments is still
allowed.

=head2 In double-quotish C<\cI<X>>, I<X> must now be a printable ASCII character

In prior releases, failure to do this raised a deprecation warning.

=head2 Splitting the tokens C<(?> and C<(*> in regular expressions is
now a fatal compilation error.

These had been deprecated since v5.18.

=head2 C<qr/foo/x> now ignores all Unicode pattern white space

The C</x> regular expression modifier allows the pattern to contain
white space and comments (both of which are ignored) for improved
readability.  Until now, not all the white space characters that Unicode
designates for this purpose were handled.  The additional ones now
recognized are

    U+0085 NEXT LINE
    U+200E LEFT-TO-RIGHT MARK
    U+200F RIGHT-TO-LEFT MARK
    U+2028 LINE SEPARATOR
    U+2029 PARAGRAPH SEPARATOR

The use of these characters with C</x> outside bracketed character
classes and when not preceded by a backslash has raised a deprecation
warning since v5.18.  Now they will be ignored.

=head2 Comment lines within S<C<(?[ ])>> are now ended only by a C<\n>

S<C<(?[ ])>>  is an experimental feature, introduced in v5.18.  It operates
as if C</x> is always enabled.  But there was a difference: comment
lines (following a C<#> character) were terminated by anything matching
C<\R> which includes all vertical whitespace, such as form feeds.  For
consistency, this is now changed to match what terminates comment lines
outside S<C<(?[ ])>>, namely a C<\n> (even if escaped), which is the
same as what terminates a heredoc string and formats.

=head2 C<(?[...])> operators now follow standard Perl precedence

This experimental feature allows set operations in regular expression patterns.
Prior to this, the intersection operator had the same precedence as the other
binary operators.  Now it has higher precedence.  This could lead to different
outcomes than existing code expects (though the documentation has always noted
that this change might happen, recommending fully parenthesizing the
expressions).  See L<perlrecharclass/Extended Bracketed Character Classes>.

=head2 Omitting C<%> and C<@> on hash and array names is no longer permitted

Really old Perl let you omit the C<@> on array names and the C<%> on hash
names in some spots.  This has issued a deprecation warning since Perl
5.000, and is no longer permitted.

=head2 C<"$!"> text is now in English outside C<"use locale"> scope

Previously, the text, unlike almost everything else, always came out
based on the current underlying locale of the program.  (Also affected
on some systems is C<"$^E>".)  For programs that are unprepared to
handle locale, this can cause garbage text to be displayed.  It's better
to display text that is translatable via some tool than garbage text
which is much harder to figure out.

=head2 C<"$!"> text will be returned in UTF-8 when appropriate

The stringification of C<$!> and C<$^E> will have the UTF-8 flag set
when the text is actually non-ASCII UTF-8.  This will enable programs
that are set up to be locale-aware to properly output messages in the
user's native language.  Code that needs to continue the 5.20 and
earlier behavior can do the stringification within the scopes of both
S<C<'use bytes'>> and S<C<'use locale ":messages">>.  No other Perl
operations will
be affected by locale; only C<$!> and C<$^E> stringification.  The
'bytes' pragma causes the UTF-8 flag to not be set, just as in previous
Perl releases.  This resolves
L<[perl #112208]|https://rt.perl.org/Ticket/Display.html?id=112208>.

=head2 Support for C<?PATTERN?> without explicit operator has been removed

Starting regular expressions matching only once directly with the
question mark delimiter is now a syntax error, so that the question mark
can be available for use in new operators.  Write C<m?PATTERN?> instead,
explicitly using the C<m> operator: the question mark delimiter still
invokes match-once behaviour.

=head2 C<defined(@array)> and C<defined(%hash)> are now fatal errors

These have been deprecated since v5.6.1 and have raised deprecation
warnings since v5.16.

=head2 Using a hash or an array as a reference are now fatal errors

For example, C<< %foo->{"bar"} >> now causes a fatal compilation
error.  These have been deprecated since before v5.8, and have raised
deprecation warnings since then.

=head2 Changes to the C<*> prototype

The C<*> character in a subroutine's prototype used to allow barewords to take
precedence over most, but not all, subroutine names.  It was never
consistent and exhibited buggy behaviour.

Now it has been changed, so subroutines always take precedence over barewords,
which brings it into conformity with similarly prototyped built-in functions:

    sub splat(*) { ... }
    sub foo { ... }
    splat(foo); # now always splat(foo())
    splat(bar); # still splat('bar') as before
    close(foo); # close(foo())
    close(bar); # close('bar')

=head1 Deprecations

=head2 Setting C<${^ENCODING}> to anything but C<undef>

This variable allows Perl scripts to be written in a non-ASCII,
non-UTF-8 encoding.  However, it affects all modules globally, leading
to wrong answers and segmentation faults.  New scripts should be written
in UTF-8; old scripts should be converted to UTF-8, which is easily done
with the L<encoding> pragma.

=head2 Use of non-graphic characters in single-character variable names

The syntax for single-character variable names is more lenient than
for longer variable names, allowing the one-character name to be a
punctuation character or even invisible (a non-graphic).  Perl v5.20
deprecated the ASCII-range controls as such a name.  Now, all
non-graphic characters that formerly were allowed are deprecated.
The practical effect of this occurs only when not under C<S<"use
utf8">>, and affects just the C1 controls (code points 0x80 through
0xFF), NO-BREAK SPACE, and SOFT HYPHEN.

=head2 Inlining of C<sub () { $var }> with observable side-effects

In many cases Perl makes S<C<sub () { $var }>> into an inlinable constant
subroutine, capturing the value of C<$var> at the time the C<sub> expression
is evaluated.  This can break the closure behaviour in those cases where
C<$var> is subsequently modified, since the subroutine won't return the
changed value. (Note that this all only applies to anonymous subroutines
with an empty prototype (C<sub ()>).)

This usage is now deprecated in those cases where the variable could be
modified elsewhere.  Perl detects those cases and emits a deprecation
warning.  Such code will likely change in the future and stop producing a
constant.

If your variable is only modified in the place where it is declared, then
Perl will continue to make the sub inlinable with no warnings.

    sub make_constant {
        my $var = shift;
        return sub () { $var }; # fine
    }

    sub make_constant_deprecated {
        my $var;
        $var = shift;
        return sub () { $var }; # deprecated
    }

    sub make_constant_deprecated2 {
        my $var = shift;
        log_that_value($var); # could modify $var
        return sub () { $var }; # deprecated
    }

In the second example above, detecting that C<$var> is assigned to only once
is too hard to detect.  That it happens in a spot other than the C<my>
declaration is enough for Perl to find it suspicious.

This deprecation warning happens only for a simple variable for the body of
the sub.  (A C<BEGIN> block or C<use> statement inside the sub is ignored,
because it does not become part of the sub's body.)  For more complex
cases, such as S<C<sub () { do_something() if 0; $var }>> the behaviour has
changed such that inlining does not happen if the variable is modifiable
elsewhere.  Such cases should be rare.

=head2 Use of multiple /x regexp modifiers

It is now deprecated to say something like any of the following:

    qr/foo/xx;
    /(?xax:foo)/;
    use re qw(/amxx);

That is, now C<x> should only occur once in any string of contiguous
regular expression pattern modifiers.  We do not believe there are any
occurrences of this in all of CPAN.  This is in preparation for a future
Perl release having C</xx> mean to allow white-space for readability in
bracketed character classes (those enclosed in square brackets:
C<[...]>).

=head2 Using a NO-BREAK space in a character alias for C<\N{...}> is now
deprecated

This non-graphic character is essentially indistinguishable from a
regular space, and so should not be allowed.  See
L<charnames/CUSTOM ALIASES>.

=head2 A literal C<"{"> should now be escaped in a pattern

If you want a literal left curly bracket (also called a left brace) in a
regular expression pattern, you should now escape it by either
preceding it with a backslash (C<"\{">) or enclosing it within square
brackets C<"[{]">, or by using C<\Q>; otherwise a deprecation warning
will be raised.  This was first announced as forthcoming in the v5.16
release; it will allow future extensions to the language to happen.

=head2 Making all warnings fatal is discouraged

The documentation for L<fatal warnings|warnings/Fatal Warnings> notes that
C<< use warnings FATAL => 'all' >> is discouraged and provides stronger
language about the risks of fatal warnings in general.

=head1 Performance Enhancements

=over 4

=item *

If a method or class name is known at compile time, a hash is precomputed
to speed up run-time method lookup.  Also, compound method names like
C<SUPER::new> are parsed at compile time, to save having to parse them at
run time.

=item *

Array and hash lookups (especially nested ones) that use only constants
or simple variables as keys, are now considerably faster. See
L</Internal Changes> for more details.

=item *

C<(...)x1>, C<("constant")x0> and C<($scalar)x0> are now optimised in list
context.  If the right-hand argument is a constant 1, the repetition
operator disappears.  If the right-hand argument is a constant 0, the whole
expression is optimised to the empty list, so long as the left-hand
argument is a simple scalar or constant.  C<(foo())x0> is not optimised.

=item *

C<substr> assignment is now optimised into 4-argument C<substr> at the end
of a subroutine (or as the argument to C<return>).  Previously, this
optimisation only happened in void context.

=item *

Assignment to lexical variables is now more often optimised away.  For
instance, in
C<$lexical = chr $foo>, the C<chr> operator writes directly to the lexical
variable instead of returning a value that gets copied.  This optimisation
has been extended to C<split>, C<x> and C<vec> on the right-hand side.  It
has also been made to work with state variable initialization.

=item *

In C<"\L...">, C<"\Q...">, etc., the extra "stringify" op is now optimised
away, making these just as fast as C<lcfirst>, C<quotemeta>, etc.

=item *

Assignment to an empty list is now sometimes faster.  In particular, it
never calls C<FETCH> on tied arguments on the right-hand side, whereas it
used to sometimes.

=item *

C<length> is up to 20% faster for non-magical/non-tied scalars containing a
string if it is a non-utf8 string or if is in scope of C<use bytes>.

=item *

On most perl builds with 64 bit integers, non-magical/non-tied scalars
that contain only a floating point value now use between 8 and 32 less bytes
of memory, depending on OS.

=item *

In C<@array = split>, the assignment can be optimized away with C<split>
writing directly to the array.  This optimisation was happening only for
package arrays other than C<@_>, and only sometimes.  Now this
optimisation happens almost all the time.

=item *

C<join> is now subject to constant folding.  So for example
C<join "-", "a", "b"> is converted at compile-time to C<"a-b">.
Moreover, C<join> with a scalar or constant for the separator and a
single-item list to join is simplified to a stringification.  The
separator doesn't even get evaluated.

=item *

C<qq(@array)> is implemented using two ops: a stringify op and a join op.
If the C<qq> contains nothing but a single array, the stringification is
optimized away.

=item *

S<C<our $var>> and S<C<our($s,@a,%h)>> in void context are no longer evaluated at
run time.  Even a whole sequence of S<C<our $foo;>> statements will simply be
skipped over.  The same applies to C<state> variables.

=item *

Many internal functions have been refactored to improve performance and reduce
their memory footprints.
L<[perl #121436]|https://rt.perl.org/Ticket/Display.html?id=121436>
L<[perl #121906]|https://rt.perl.org/Ticket/Display.html?id=121906>
L<[perl #121969]|https://rt.perl.org/Ticket/Display.html?id=121969>

=item *

C<-T> and C<-B> filetests will return sooner when an empty file is detected.
L<[perl #121489]|https://rt.perl.org/Ticket/Display.html?id=121489>

=item *

Hash lookups where the key is a constant are faster.

=item *

Subroutines with an empty prototype and bodies containing just C<undef> are now
eligible for inlining.
L<[perl #122728]|https://rt.perl.org/Ticket/Display.html?id=122728>

=item *

Subroutines in packages no longer need to be stored in typeglobs:
declaring a subroutine will now put a simple sub reference directly in the
stash if possible, saving memory.  The typeglob still notionally exists,
so accessing it will cause the stash entry to be upgraded to a typeglob
(i.e. this is just an internal implementation detail).
This optimization does not currently apply to XSUBs or exported
subroutines, and method calls will undo it, since they cache things in
typeglobs.
L<[perl #120441]|https://rt.perl.org/Ticket/Display.html?id=120441>

=item *

The functions C<utf8::native_to_unicode()> and C<utf8::unicode_to_native()>
(see L<utf8>) are now optimized out on ASCII platforms.  There is now not even
a minimal performance hit in writing code portable between ASCII and EBCDIC
platforms.

=item *

Win32 Perl uses 8 KB less of per-process memory than before for every perl
process, because some data is now memory mapped from disk and shared
between perl processes from the same perl binary.

=back

=head1 Modules and Pragmata

XXX All changes to installed files in F<cpan/>, F<dist/>, F<ext/> and F<lib/>
go here.  If Module::CoreList is updated, generate an initial draft of the
following sections using F<Porting/corelist-perldelta.pl>.  A paragraph summary
for important changes should then be added by hand.  In an ideal world,
dual-life modules would have a F<Changes> file that could be cribbed.

[ Within each section, list entries as a =item entry ]

=head2 New Modules and Pragmata

=over 4

=item *

XXX

=back

=head2 Updated Modules and Pragmata

=over 4

=item *

L<ExtUtils::MakeMaker> has been upgraded from version 6.98 to 7.04_01.

Some changes from github have been backported to prevent failures and
noise on Win32 when C<chcp> is missing or unavailable.
L<[perl #123998]|https://rt.perl.org/Ticket/Display.html?id=123998>

=back

=head2 Removed Modules and Pragmata

=over 4

=item *

XXX

=back

=head1 Documentation

=head2 New Documentation

=head3 L<perlunicook>

This document, by Tom Christiansen, provides examples of handling Unicode in
Perl.

=head2 Changes to Existing Documentation

=head3 L<perlapi>

=over 4

=item *

Note that C<SvSetSV> doesn't do set magic.

=item *

C<sv_usepvn_flags> - Fix documentation to mention the use of C<NewX> instead of
C<malloc>.

L<[perl #121869]|https://rt.perl.org/Ticket/Display.html?id=121869>

=item *

Clarify where C<NUL> may be embedded or is required to terminate a string.

=item *

Previously missing documentation due to formatting errors are now included.

=item *

Entries are now organized into groups rather than by file where they are found.

=item *

Alphabetical sorting of entries is now handled by the POD generator to make
entries easier to find when scanning.

=back

=head3 L<perldata>

=over 4

=item *

The syntax of single-character variable names has been brought
up-to-date and more fully explained.

=back

=head3 L<perlebcdic>

=over 4

=item *

This document has been significantly updated in the light of recent
improvements to EBCDIC support.

=back

=head3 L<perlfunc>

=over 4

=item *

Mention that C<study()> is currently a no-op.

=item *

Calling C<delete> or C<exists> on array values is now described as "strongly
discouraged" rather than "deprecated".

=item *

Improve documentation of C<< our >>.

=item *

C<-l> now notes that it will return false if symlinks aren't supported by the
file system.

L<[perl #121523]|https://rt.perl.org/Ticket/Display.html?id=121523>

=item *

Note that C<exec LIST> and C<system LIST> may fall back to the shell on
Win32. Only C<exec PROGRAM LIST> and C<system PROGRAM LIST> indirect object
syntax will reliably avoid using the shell.

This has also been noted in L<perlport>.

L<[perl #122046]|https://rt.perl.org/Ticket/Display.html?id=122046>

=back

=head3 L<perlguts>

=over 4

=item *

The OOK example has been updated to account for COW changes and a change in the
storage of the offset.

=item *

Details on C level symbols and libperl.t added.

=item *

Information on Unicode handling has been added

=item *

Information on EBCDIC handling has been added

=back

=head3 L<perlhacktips>

=over 4

=item *

Documentation has been added illustrating the perils of assuming the contents
of static memory pointed to by the return values of Perl wrappers for C library
functions doesn't change.

=item *

Recommended replacements for tmpfile, atoi, strtol, and strtoul added.

=item *

Updated documentation for the C<test.valgrind> C<make> target.

L<[perl #121431]|https://rt.perl.org/Ticket/Display.html?id=121431>

=back

=head3 L<perlmodstyle>

=over 4

=item *

Instead of pointing to the module list, we are now pointing to
L<PrePAN|http://prepan.org/>.

=back

=head3 L<perlpolicy>

=over 4

=item *

We now have a code of conduct for the I<< p5p >> mailing list, as documented
in L<< perlpolicy/STANDARDS OF CONDUCT >>.

=item *

The conditions for marking an experimental feature as non-experimental are now
set out.

=back

=head3 L<perlport>

=over 4

=item *

Out-of-date VMS-specific information has been fixed/simplified.

=item *

Notes about EBCDIC have been added.

=back

=head3 L<perlre>

=over 4

=item *

The C</x> modifier has been clarified to note that comments cannot be continued
onto the next line by escaping them.

=back

=head3 L<perlrebackslash>

=over 4

=item *

Added documentation of C<\b{sb}>, C<\b{wb}>, C<\b{gcb}>, and C<\b{g}>.

=back

=head3 L<perlrecharclass>

=over 4

=item *

Clarifications have been added to L<perlrecharclass/Character Ranges>
to the effect that Perl guarantees that C<[A-Z]>, C<[a-z]>, C<[0-9]> and
any subranges thereof in regular expression bracketed character classes
are guaranteed to match exactly what a naive English speaker would
expect them to match, even on platforms (such as EBCDIC) where special
handling is required to accomplish this.

=item *

The documentation of Bracketed Character Classes has been expanded to cover the
improvements in C<qr/[\N{named sequence}]/> (see under L</Selected Bug Fixes>).

=back

=head3 L<perlsec>

=over 4

=item *

Comments added on algorithmic complexity and tied hashes.

=back

=head3 L<perlsyn>

=over 4

=item *

An ambiguity in the documentation of the C<...> statement has been corrected.
L<[perl #122661]|https://rt.perl.org/Ticket/Display.html?id=122661>

=item *

The empty conditional in C<< for >> and C<< while >> is now documented
in L<< perlsyn >>.

=back

=head3 L<perlunicode>

=over 4

=item *

This has had extensive revisions to bring it up-to-date with current
Unicode support and to make it more readable.

=back

=head3 L<perluniintro>

=over 4

=item *

Advice for how to make sure your strings and regular expression patterns are
interpreted as Unicode has been updated.

=back

=head3 L<perlvar>

=over 4

=item *

Further clarify version number representations and usage.

=back

=head3 L<perlvms>

=over 4

=item *

Out-of-date and/or incorrect material has been removed.

=item *

Updated documentation on environment and shell interaction in VMS.

=back

=head3 L<perlxs>

=over 4

=item *

Added a discussion of locale issues in XS code.

=back

=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 4

=item *

L<Bad symbol for scalar|perldiag/"Bad symbol for scalar">

(P) An internal request asked to add a scalar entry to something that
wasn't a symbol table entry.

=item *

L<Can't use a hash as a reference|perldiag/"Can't use a hash as a reference">

(F) You tried to use a hash as a reference, as in
C<< %foo->{"bar"} >> or C<< %$ref->{"hello"} >>.  Versions of perl E<lt>= 5.6.1
used to allow this syntax, but shouldn't have.

=item *

L<Can't use an array as a reference|perldiag/"Can't use an array as a reference">

(F) You tried to use an array as a reference, as in
C<< @foo->[23] >> or C<< @$ref->[99] >>.  Versions of perl E<lt>= 5.6.1 used to
allow this syntax, but shouldn't have.

=item *

L<Can't use 'defined(@array)' (Maybe you should just omit the defined()?)|perldiag/"Can't use 'defined(@array)' (Maybe you should just omit the defined()?)">

(F) C<defined()> is not useful on arrays because it
checks for an undefined I<scalar> value.  If you want to see if the
array is empty, just use S<C<if (@array) { # not empty }>> for example.

=item *

L<Can't use 'defined(%hash)' (Maybe you should just omit the defined()?)|perldiag/"Can't use 'defined(%hash)' (Maybe you should just omit the defined()?)">

(F) C<defined()> is not usually right on hashes.

Although S<C<defined %hash>> is false on a plain not-yet-used hash, it
becomes true in several non-obvious circumstances, including iterators,
weak references, stash names, even remaining true after S<C<undef %hash>>.
These things make S<C<defined %hash>> fairly useless in practice, so it now
generates a fatal error.

If a check for non-empty is what you wanted then just put it in boolean
context (see L<perldata/Scalar values>):

    if (%hash) {
       # not empty
    }

If you had S<C<defined %Foo::Bar::QUUX>> to check whether such a package
variable exists then that's never really been reliable, and isn't
a good way to enquire about the features of a package, or whether
it's loaded, etc.

=item *

L<Cannot chr %f|perldiag/"Cannot chr %f">

(F) You passed an invalid number (like an infinity or not-a-number) to
C<chr>.

=item *

L<Cannot compress %f in pack|perldiag/"Cannot compress %f in pack">

(F) You tried converting an infinity or not-a-number to an unsigned
character, which makes no sense.

=item *

L<Cannot pack %f with '%c'|perldiag/"Cannot pack %f with '%c'">

(F) You tried converting an infinity or not-a-number to a character,
which makes no sense.

=item *

L<Cannot print %f with '%c'|perldiag/"Cannot printf %f with '%c'">

(F) You tried printing an infinity or not-a-number as a character (C<%c>),
which makes no sense.  Maybe you meant C<'%s'>, or just stringifying it?

=item *

L<charnames alias definitions may not contain a sequence of multiple spaces|perldiag/"charnames alias definitions may not contain a sequence of multiple spaces">

(F) You defined a character name which had multiple space
characters in a row.  Change them to single spaces.  Usually these
names are defined in the C<:alias> import argument to C<use charnames>, but
they could be defined by a translator installed into C<$^H{charnames}>.
See L<charnames/CUSTOM ALIASES>.

=item *

L<charnames alias definitions may not contain trailing white-space|perldiag/"charnames alias definitions may not contain trailing white-space">

(F) You defined a character name which ended in a space
character.  Remove the trailing space(s).  Usually these names are
defined in the C<:alias> import argument to C<use charnames>, but they
could be defined by a translator installed into C<$^H{charnames}>.
See L<charnames/CUSTOM ALIASES>.

=item *

L<:const is not permitted on named subroutines|perldiag/":const is not permitted on named subroutines">

(F) The "const" attribute causes an anonymous subroutine to be run and
its value captured at the time that it is cloned.  Names subroutines are
not cloned like this, so the attribute does not make sense on them.

=item *

L<Hexadecimal float: internal error|perldiag/"Hexadecimal float: internal error">

(F) Something went horribly bad in hexadecimal float handling.

=item *

L<Hexadecimal float: unsupported long double format|perldiag/"Hexadecimal float: unsupported long double format">

(F) You have configured Perl to use long doubles but
the internals of the long double format are unknown,
therefore the hexadecimal float output is impossible.

=item *

L<Illegal suidscript|perldiag/"Illegal suidscript">

(F) The script run under suidperl was somehow illegal.

=item *

L<In '(?...)', the '(' and '?' must be adjacent in regex; marked by S<<-- HERE> in mE<sol>%sE<sol>|perldiag/"In '(?...)', the '(' and '?' must be adjacent in regex; marked by <-- HERE in m/%s/">

(F) The two-character sequence C<"(?"> in
this context in a regular expression pattern should be an
indivisible token, with nothing intervening between the C<"(">
and the C<"?">, but you separated them.

=item *

L<In '(*VERB...)', the '(' and '*' must be adjacent in regex; marked by S<<-- HERE> in mE<sol>%sE<sol>|perldiag/"In '(*VERB...)', the '(' and '*' must be adjacent in regex; marked by <-- HERE in m/%s/">

(F) The two-character sequence C<"(*"> in
this context in a regular expression pattern should be an
indivisible token, with nothing intervening between the C<"(">
and the C<"*">, but you separated them.

=item *

L<Invalid quantifier in {,} in regex; marked by <-- HERE in mE<sol>%sE<sol>|perldiag/"Invalid quantifier in {,} in regex; marked by <-- HERE in m/%s/">

(F) The pattern looks like a {min,max} quantifier, but the min or max could not
be parsed as a valid number - either it has leading zeroes, or it represents
too big a number to cope with.  The S<<-- HERE> shows where in the regular
expression the problem was discovered.  See L<perlre>.

=back

=head3 New Warnings

=over 4

=item *

L<\C is deprecated in regex|perldiag/"\C is deprecated in regex; marked by <-- HERE in m/%s/">

(D deprecated) The C<< /\C/ >> character class was deprecated in v5.20, and
now emits a warning. It is intended that it will become an error in v5.24.
This character class matches a single byte even if it appears within a
multi-byte character, breaks encapsulation, and can corrupt utf8
strings.

=item *

L<'%s' is an unknown bound type in regex|perldiag/"'%s' is an unknown bound type in regex; marked by <-- HERE in m/%s/">

You used C<\b{...}> or C<\B{...}> and the C<...> is not known to
Perl.  The current valid ones are given in
L<perlrebackslash/\b{}, \b, \B{}, \B>.

=item *

L<"%s" is more clearly written simply as "%s" in regex; marked by E<lt>-- HERE in mE<sol>%sE<sol>|perldiag/"%s" is more clearly written simply as "%s" in regex; marked by <-- HERE in mE<sol>%sE<sol>>

(W regexp) (only under C<S<use re 'strict'>> or within C<(?[...])>)

You specified a character that has the given plainer way of writing it,
and which is also portable to platforms running with different character
sets.

=item *

L<Argument "%s" treated as 0 in increment (++)|perldiag/"Argument "%s" treated
as 0 in increment (++)">

(W numeric) The indicated string was fed as an argument to the C<++> operator
which expects either a number or a string matching C</^[a-zA-Z]*[0-9]*\z/>.
See L<perlop/Auto-increment and Auto-decrement> for details.

=item *

L<Both or neither range ends should be Unicode in regex; marked by E<lt>-- HERE in mE<sol>%sE<sol>|perldiag/"Both or neither range ends should be Unicode in regex; marked by <-- HERE in m/%s/">

(W regexp) (only under C<S<use re 'strict'>> or within C<(?[...])>)

In a bracketed character class in a regular expression pattern, you
had a range which has exactly one end of it specified using C<\N{}>, and
the other end is specified using a non-portable mechanism.  Perl treats
the range as a Unicode range, that is, all the characters in it are
considered to be the Unicode characters, and which may be different code
points on some platforms Perl runs on.  For example, C<[\N{U+06}-\x08]>
is treated as if you had instead said C<[\N{U+06}-\N{U+08}]>, that is it
matches the characters whose code points in Unicode are 6, 7, and 8.
But that C<\x08> might indicate that you meant something different, so
the warning gets raised.

=item *

L<:const is experimental|perldiag/":const is experimental">

(S experimental::const_attr) The "const" attribute is experimental.
If you want to use the feature, disable the warning with C<no warnings
'experimental::const_attr'>, but know that in doing so you are taking
the risk that your code may break in a future Perl version.

=item *

L<gmtime(%f) failed|perldiag/"gmtime(%f) failed">

(W overflow) You called C<gmtime> with a number that it could not handle:
too large, too small, or NaN.  The returned value is C<undef>.

=item *

L<Hexadecimal float: exponent overflow|perldiag/"Hexadecimal float: exponent overflow">

(W overflow) The hexadecimal floating point has larger exponent
than the floating point supports.

=item *

L<Hexadecimal float: exponent underflow|perldiag/"Hexadecimal float: exponent underflow">

(W overflow) The hexadecimal floating point has smaller exponent
than the floating point supports.

=item *

L<Hexadecimal float: mantissa overflow|perldiag/"Hexadecimal float: mantissa overflow">

(W overflow) The hexadecimal floating point literal had more bits in
the mantissa (the part between the 0x and the exponent, also known as
the fraction or the significand) than the floating point supports.

=item *

L<Hexadecimal float: precision loss|perldiag/"Hexadecimal float: precision loss">

(W overflow) The hexadecimal floating point had internally more
digits than could be output.  This can be caused by unsupported
long double formats, or by 64-bit integers not being available
(needed to retrieve the digits under some configurations).

=item *

L<localtime(%f) failed|perldiag/"localtime(%f) failed">

(W overflow) You called C<localtime> with a number that it could not handle:
too large, too small, or NaN.  The returned value is C<undef>.

=item *

L<Negative repeat count does nothing|perldiag/"Negative repeat count does nothing">

(W numeric) You tried to execute the
L<C<x>|perlop/Multiplicative Operators> repetition operator fewer than 0
times, which doesn't make sense.

=item *

L<NO-BREAK SPACE in a charnames alias definition is deprecated|perldiag/"NO-BREAK SPACE in a charnames alias definition is deprecated">

(D deprecated) You defined a character name which contained a no-break
space character.  Change it to a regular space.  Usually these names are
defined in the C<:alias> import argument to C<use charnames>, but they
could be defined by a translator installed into C<$^H{charnames}>.  See
L<charnames/CUSTOM ALIASES>.

=item *

L<Non-finite repeat count does nothing|perldiag/"Non-finite repeat count does nothing">

(W numeric) You tried to execute the
L<C<x>|perlop/Multiplicative Operators> repetition operator C<Inf> (or
C<-Inf>) or C<NaN> times, which doesn't make sense.

=item *

L<PerlIO layer ':win32' is experimental|perldiag/"PerlIO layer ':win32' is experimental">

(S experimental::win32_perlio) The C<:win32> PerlIO layer is
experimental.  If you want to take the risk of using this layer,
simply disable this warning:

    no warnings "experimental::win32_perlio";

=item *

L<Ranges of ASCII printables should be some subset of "0-9", "A-Z", or "a-z" in regex; marked by E<lt>-- HERE in mE<sol>%sE<sol>|perldiag/"Ranges of ASCII printables should be some subset of "0-9", "A-Z", or "a-z" in regex; marked by <-- HERE in mE<sol>%sE<sol>">

(W regexp) (only under C<S<use re 'strict'>> or within C<(?[...])>)

Stricter rules help to find typos and other errors.  Perhaps you didn't
even intend a range here, if the C<"-"> was meant to be some other
character, or should have been escaped (like C<"\-">).  If you did
intend a range, the one that was used is not portable between ASCII and
EBCDIC platforms, and doesn't have an obvious meaning to a casual
reader.

 [3-7]    # OK; Obvious and portable
 [d-g]    # OK; Obvious and portable
 [A-Y]    # OK; Obvious and portable
 [A-z]    # WRONG; Not portable; not clear what is meant
 [a-Z]    # WRONG; Not portable; not clear what is meant
 [%-.]    # WRONG; Not portable; not clear what is meant
 [\x41-Z] # WRONG; Not portable; not obvious to non-geek

(You can force portability by specifying a Unicode range, which means that
the endpoints are specified by
L<C<\N{...}>|perlrecharclass/Character Ranges>, but the meaning may
still not be obvious.)
The stricter rules require that ranges that start or stop with an ASCII
character that is not a control have all their endpoints be a literal
character, and not some escape sequence (like C<"\x41">), and the ranges
must be all digits, or all uppercase letters, or all lowercase letters.

=item *

L<Ranges of digits should be from the same group in regex; marked by E<lt>-- HERE in mE<sol>%sE<sol>|perldiag/"Ranges of digits should be from the same group in regex; marked by <-- HERE in m/%s/">

(W regexp) (only under C<S<use re 'strict'>> or within C<(?[...])>)

Stricter rules help to find typos and other errors.  You included a
range, and at least one of the end points is a decimal digit.  Under the
stricter rules, when this happens, both end points should be digits in
the same group of 10 consecutive digits.

=item *

L<Redundant argument in %s|perldiag/Redundant argument in %s>

(W redundant) You called a function with more arguments than other
arguments you supplied indicated would be needed. Currently only
emitted when a printf-type format required fewer arguments than were
supplied, but might be used in the future for e.g. L<perlfunc/pack>.

The warnings category C<< redundant >> is new. See also
L<[perl #121025]|https://rt.perl.org/Ticket/Display.html?id=121025>.

=item *

L<Use of \b{} for non-UTF-8 locale is wrong.  Assuming a UTF-8 locale|perldiag/"Use of \b{} for non-UTF-8 locale is wrong.  Assuming a UTF-8 locale">

You are matching a regular expression using locale rules,
and a Unicode boundary is being matched, but the locale is not a Unicode
one.  This doesn't make sense.  Perl will continue, assuming a Unicode
(UTF-8) locale, but the results could well be wrong except if the locale
happens to be ISO-8859-1 (Latin1) where this message is spurious and can
be ignored.

=item *

L<< Using E<sol>u for '%s' instead of E<sol>%s in regex; marked by E<lt>-- HERE in mE<sol>%sE<sol>|perldiag/"Using E<sol>u for '%s' instead of E<sol>%s in regex; marked by <-- HERE in mE<sol>%sE<sol>" >>

You used a Unicode boundary (C<\b{...}> or C<\B{...}>) in a
portion of a regular expression where the character set modifiers C</a>
or C</aa> are in effect.  These two modifiers indicate an ASCII
interpretation, and this doesn't make sense for a Unicode definition.
The generated regular expression will compile so that the boundary uses
all of Unicode.  No other portion of the regular expression is affected.

=item *

L<The bitwise feature is experimental|perldiag/"The bitwise feature is experimental">

This warning is emitted if you use bitwise
operators (C<& | ^ ~ &. |. ^. ~.>) with the "bitwise" feature enabled.
Simply suppress the warning if you want to use the feature, but know
that in doing so you are taking the risk of using an experimental
feature which may change or be removed in a future Perl version:

    no warnings "experimental::bitwise";
    use feature "bitwise";
    $x |.= $y;

=item *

L<Unescaped left brace in regex is deprecated, passed through in regex; marked by <-- HERE in mE<sol>%sE<sol>|perldiag/"Unescaped left brace in regex is deprecated, passed through in regex; marked by <-- HERE in m/%s/">

(D deprecated, regexp) You used a literal C<"{"> character in a regular
expression pattern. You should change to use C<"\{"> instead, because a future
version of Perl (tentatively v5.26) will consider this to be a syntax error.  If
the pattern delimiters are also braces, any matching right brace
(C<"}">) should also be escaped to avoid confusing the parser, for
example,

    qr{abc\{def\}ghi}

=item *

L<Use of literal non-graphic characters in variable names is deprecated|perldiag/"Use of literal non-graphic characters in variable names is deprecated">

=item *

L<Useless use of attribute "const"|perldiag/Useless use of attribute "const">

(W misc) The "const" attribute has no effect except
on anonymous closure prototypes.  You applied it to
a subroutine via L<attributes.pm|attributes>.  This is only useful
inside an attribute handler for an anonymous subroutine.

=item *

L<E<quot>use re 'strict'E<quot> is experimental|perldiag/"use re 'strict'" is experimental>

(S experimental::re_strict) The things that are different when a regular
expression pattern is compiled under C<'strict'> are subject to change
in future Perl releases in incompatible ways.  This means that a pattern
that compiles today may not in a future Perl release.  This warning is
to alert you to that risk.

=item *

L<Warning: unable to close filehandle properly: %s|perldiag/"Warning: unable to close filehandle properly: %s">

L<Warning: unable to close filehandle %s properly: %s|perldiag/"Warning: unable to close filehandle %s properly: %s">

(S io) An error occurred when Perl implicitly closed a filehandle.  This
usually indicates your file system ran out of disk space.

=item *

L<Wide character (U+%X) in %s|perldiag/"Wide character (U+%X) in %s">

(W locale) While in a single-byte locale (I<i.e.>, a non-UTF-8
one), a multi-byte character was encountered.   Perl considers this
character to be the specified Unicode code point.  Combining non-UTF8
locales and Unicode is dangerous.  Almost certainly some characters
will have two different representations.  For example, in the ISO 8859-7
(Greek) locale, the code point 0xC3 represents a Capital Gamma.  But so
also does 0x393.  This will make string comparisons unreliable.

You likely need to figure out how this multi-byte character got mixed up
with your single-byte locale (or perhaps you thought you had a UTF-8
locale, but Perl disagrees).

=item *

The following two warnings for C<tr///> used to be skipped if the
transliteration contained wide characters, but now they occur regardless of
whether there are wide characters or not:

L<Useless use of E<sol>d modifier in transliteration operator|perldiag/"Useless use of /d modifier in transliteration operator">

L<Replacement list is longer than search list|perldiag/Replacement list is longer than search list>

=item *

A new C<locale> warning category has been created, with the following warning
messages currently in it:

=over 4

=item *

L<Locale '%s' may not work well.%s|perldiag/Locale '%s' may not work well.%s>

=item *

L<Can't do %s("%s") on non-UTF-8 locale; resolved to "%s".|perldiag/Can't do %s("%s") on non-UTF-8 locale; resolved to "%s".>

=back

=back

=head2 Changes to Existing Diagnostics

=over 4

=item *

<> should be quotes

This warning has been changed to
L<< <> at require-statement should be quotes|perldiag/"<> at require-statement should be quotes" >>
to make the issue more identifiable.

=item *

L<Argument "%s" isn't numeric%s|perldiag/"Argument "%s" isn't numeric%s">
now adds the following note:

 Note that for the Inf and NaN (infinity and not-a-number) the
 definition of "numeric" is somewhat unusual: the strings themselves
 (like "Inf") are considered numeric, and anything following them is
 considered non-numeric.

=item *

L<Global symbol "%s" requires explicit package name|perldiag/"Global symbol "%s" requires explicit package name (did you forget to declare "my %s"?)">

This message has had '(did you forget to declare "my %s"?)' appended to it, to
make it more helpful to new Perl programmers.
L<[perl #121638]|https://rt.perl.org/Ticket/Display.html?id=121638>

=item *

'"my" variable &foo::bar can't be in a package' has been reworded to say
'subroutine' instead of 'variable'.

=item *

L<\N{} in character class restricted to one character in regex; marked by S<<-- HERE> in mE<sol>%sE<sol>|perldiag/"\N{} in inverted character class or as a range end-point is restricted to one character in regex; marked by <-- HERE in m/%s/">

This message has had 'character class' changed to 'inverted character class or
as a range end-point is' to reflect improvements in C<qr/[\N{named sequence}]/>
(see under L</Selected Bug Fixes>).

=item *

L<panic: frexp|perldiag/"panic: frexp: %f">

This message has had ': C<%f>' appended to it, to show what the offending floating
point number is.

=item *

B<Possible precedence problem on bitwise %c operator> reworded as
L<Possible precedence problem on bitwise %s operator|perldiag/"Possible precedence problem on bitwise %s operator">.

=item *

C<require> with no argument or undef used to warn about a Null filename; now
it dies with C<Missing or undefined argument to require>.

=item *

L<Unsuccessful %s on filename containing newline|perldiag/"Unsuccessful %s on filename containing newline">

This warning is now only produced when the newline is at the end of
the filename.

=item *

"Variable C<%s> will not stay shared" has been changed to say "Subroutine"
when it is actually a lexical sub that will not stay shared.

=item *

L<Variable length lookbehind not implemented in regex mE<sol>%sE<sol>|perldiag/"Variable length lookbehind not implemented in regex m/%s/">

Information about Unicode behaviour has been added.

=back

=head2 Diagnostic Removals

=over

=item *

"Ambiguous use of -foo resolved as -&foo()"

There is actually no ambiguity here, and this impedes the use of negated
constants; e.g., C<-Inf>.

=item *

"Constant is not a FOO reference"

Compile-time checking of constant dereferencing (e.g., C<< my_constant->() >>)
has been removed, since it was not taking overloading into account.
L<[perl #69456]|https://rt.perl.org/Ticket/Display.html?id=69456>
L<[perl #122607]|https://rt.perl.org/Ticket/Display.html?id=122607>

=back

=head1 Utility Changes

=head2 F<x2p/>

=over 4

=item *

The F<x2p/> directory has been removed from the Perl core.

This removes find2perl, s2p and a2p. They have all been released to CPAN as
separate distributions (App::find2perl, App::s2p, App::a2p).

=back

=head2 L<h2ph>

=over 4

=item *

F<h2ph> now handles hexadecimal constants in the compiler's predefined
macro definitions, as visible in C<$Config{cppsymbols}>.
L<[perl #123784]|https://rt.perl.org/Ticket/Display.html?id=123784>.

=back

=head2 L<encguess>

=over 4

=item *

No longer depends on non-core modules.

=back

=head1 Configuration and Compilation

=over 4

=item *

F<Configure> now checks for F<lrintl>, F<lroundl>, F<llrintl>, and F<llroundl>.

=item *

F<Configure> with C<-Dmksymlinks> should now be faster.
L<[perl #122002]|https://rt.perl.org/Ticket/Display.html?id=122002>.

=item *

pthreads and lcl will be linked by default if present. This allows XS modules
that require threading to work on non-threaded perls. Note that you must still
pass C<-Dusethreads> if you want a threaded perl.

=item *

For long doubles (to get more precision and range for floating point numbers)
one can now use the GCC quadmath library which implements the quadruple
precision floating point numbers on x86 and IA-64 platforms.  See
F<INSTALL> for details.

=item *

MurmurHash64A and MurmurHash64B can now be configured as the internal hash
function.

=item *

C<make test.valgrind> now supports parallel testing.

For example:

    TEST_JOBS=9 make test.valgrind

See L<perlhacktips/valgrind> for more information.

L<[perl #121431]|https://rt.perl.org/Ticket/Display.html?id=121431>

=item *

The MAD (Misc Attribute Decoration) build option has been removed

This was an unmaintained attempt at preserving
the Perl parse tree more faithfully so that automatic conversion of
Perl 5 to Perl 6 would have been easier.

This build-time configuration option had been unmaintained for years,
and had probably seriously diverged on both Perl 5 and Perl 6 sides.

=item *

A new compilation flag, C<< -DPERL_OP_PARENT >> is available. For details,
see the discussion below at L<< /Internal Changes >>.

=item *

Pathtools no longer tries to load XS on miniperl. This speeds up building perl
slightly.


=back

=head1 Testing

=over 4

=item *

F<t/porting/re_context.t> has been added to test that L<utf8> and its
dependencies only use the subset of the C<$1..$n> capture vars that
C<Perl_save_re_context()> is hard-coded to localize, because that function has no
efficient way of determining at runtime what vars to localize.

=item *

Tests for performance issues have been added in the file F<t/perf/taint.t>.

=item *

Some regular expression tests are written in such a way that they will
run very slowly if certain optimizations break. These tests have been
moved into new files, F<< t/re/speed.t >> and F<< t/re/speed_thr.t >>,
and are run with a C<< watchdog() >>.

=item *

C<< test.pl >> now allows C<< plan skip_all => $reason >>, to make it
more compatible with C<< Test::More >>.

=item *

A new test script, F<op/infnan.t>, has been added to test if Inf and NaN are
working correctly.  See L</Infinity and NaN (not-a-number) handling improved>.

=back

=head1 Platform Support

=head2 Regained Platforms

=over 4

=item IRIX and Tru64 platforms are working again.

(Some C<make test> failures remain.)

=item z/OS running EBCDIC Code Page 1047

Core perl now works on this EBCDIC platform.  Earlier perls also worked, but,
even though support wasn't officially withdrawn, recent perls would not compile
and run well.  Perl 5.20 would work, but had many bugs which have now been
fixed.  Many CPAN modules that ship with Perl still fail tests, including
Pod::Simple.  However the version of Pod::Simple currently on CPAN should work;
it was fixed too late to include in Perl 5.22.  Work is under way to fix many
of the still-broken CPAN modules, which likely will be installed on CPAN when
completed, so that you may not have to wait until Perl 5.24 to get a working
version.

=back

=head2 Discontinued Platforms

=over 4

=item NeXTSTEP/OPENSTEP

NeXTSTEP was a proprietary operating system bundled with NeXT's
workstations in the early to mid 90s; OPENSTEP was an API specification
that provided a NeXTSTEP-like environment on a non-NeXTSTEP system.  Both
are now long dead, so support for building Perl on them has been removed.

=back

=head2 Platform-Specific Notes

=over 4

=item EBCDIC

Special handling is required on EBCDIC platforms to get C<qr/[i-j]/> to
match only C<"i"> and C<"j">, since there are 7 characters between the
code points for C<"i"> and C<"j">.  This special handling had only been
invoked when both ends of the range are literals.  Now it is also
invoked if any of the C<\N{...}> forms for specifying a character by
name or Unicode code point is used instead of a literal.  See
L<perlrecharclass/Character Ranges>.

=item HP-UX

The archname now distinguishes use64bitint from use64bitall.

=item Android

Build support has been improved for cross-compiling in general and for
Android in particular.

=item VMS

=over 4

=item *

When spawning a subprocess without waiting, the return value is now
the correct PID.

=item *

Fix a prototype so linking doesn't fail under the VMS C++ compiler.

=item *

C<finite>, C<finitel>, and C<isfinite> detection has been added to
C<configure.com>, environment handling has had some minor changes, and
a fix for legacy feature checking status.

=back

=item Win32

=over 4

=item *

F<miniperl.exe> is now built with C<-fno-strict-aliasing>, allowing 64-bit
builds to complete on GCC 4.8.
L<[perl #123976]|https://rt.perl.org/Ticket/Display.html?id=123976>

=item *

C<nmake minitest> now works on Win32.  Due to dependency issues you
need to build C<nmake test-prep> first, and a small number of the
tests fail.
L<[perl #123394]|https://rt.perl.org/Ticket/Display.html?id=123394>

=item *

Perl can now be built in C++ mode on Windows by setting the makefile macro
C<USE_CPLUSPLUS> to the value "define".

=item *

List form of pipe open has been implemented for Win32.  Note: unlike
C< system LIST >> this does not fall back to the shell.
L<[perl #121159]|https://rt.perl.org/Ticket/Display.html?id=121159>

=item *

New C<DebugSymbols> and C<DebugFull> configuration options added to
Windows makefiles.

=item *

L<B> now compiles again on Windows.

=item *

Previously compiling XS modules (including CPAN ones) using Visual C++ for
Win64 resulted in around a dozen warnings per file from hv_func.h.  These
warnings have been silenced.

=item *

Support for building without PerlIO has been removed from the Windows
makefiles.  Non-PerlIO builds were all but deprecated in Perl 5.18.0 and are
already not supported by F<Configure> on POSIX systems.

=item *

Between 2 and 6 ms and 7 I/O calls have been saved per attempt to open a perl
module for each path in C<@INC>.

=item *

Intel C builds are now always built with C99 mode on.

=item *

C<%I64d> is now being used instead of C<%lld> for MinGW.

=item *

In the experimental C<:win32> layer, a crash in C<open> was fixed. Also
opening C</dev/null>, which works the Win32 Perl's normal C<:unix> layer, was
implemented for C<:win32>.
L<[perl #122224]|https://rt.perl.org/Ticket/Display.html?id=122224>

=item *

A new makefile option, C<USE_LONG_DOUBLE>, has been added to the Windows
dmake makefile for gcc builds only.  Set this to "define" if you want perl to
use long doubles to give more accuracy and range for floating point numbers.

=back

=item OpenBSD

On OpenBSD, Perl will now default to using the system C<malloc> due to the
security features it provides. Perl's own malloc wrapper has been in use
since v5.14 due to performance reasons, but the OpenBSD project believes
the tradeoff is worth it and would prefer that users who need the speed
specifically ask for it.

L<[perl #122000]|https://rt.perl.org/Ticket/Display.html?id=122000>.

=item Solaris

=over 4

=item *

We now look for the Sun Studio compiler in both F</opt/solstudio*> and
F</opt/solarisstudio*>.

=item *

Builds on Solaris 10 with C<-Dusedtrace> would fail early since make
didn't follow implied dependencies to build C<perldtrace.h>.  Added an
explicit dependency to C<depend>.
L<[perl #120120]|https://rt.perl.org/Ticket/Display.html?id=120120>

=item *

C<c99> options have been cleaned up, hints look for C<solstudio>
as well as C<SUNWspro>, and support for native C<setenv> has been added.

=back

=back

=head1 Internal Changes

=over 4

=item *

Experimental support has been added to allow ops in the optree to locate
their parent, if any. This is enabled by the non-default build option
C<-DPERL_OP_PARENT>. It is envisaged that this will eventually become
enabled by default, so XS code which directly accesses the C<op_silbing>
field of ops should be updated to be future-proofed.

On C<PERL_OP_PARENT> builds, the C<op_sibling> field has been renamed
C<op_sibparent> and a new flag, C<op_moresib>, added. On the last op in a
sibling chain, C<op_moresib> is false and C<op_sibparent> points to the
parent (if any) rather than to being C<NULL>.

To make existing code work transparently whether using C<-DPERL_OP_PARENT>
or not, a number of new macros and functions have been added that should
be used, rather than directly manipulating C<op_sibling>.

For the case of just reading C<op_sibling> to determine the next sibling,
two new macros have been added. A simple scan through a sibling chain
like this:

    for (; kid->op_sibling; kid = kid->op_sibling) { ...  }

should now be written as:

    for (; OpHAS_SIBLING(kid); kid = OpSIBLING(kid)) { ...  }

For altering optrees, A general-purpose function C<op_sibling_splice()>
has been added, which allows for manipulation of a chain of sibling ops.
By analogy with the Perl function C<splice()>, it allows you to cut out
zero or more ops from a sibling chain and replace them with zero or more
new ops.  It transparently handles all the updating of sibling, parent,
op_last pointers etc.

If you need to manipulate ops at a lower level, then three new macros,
C<OpMORESIB_set>, C<OpLASTSIB_set> and C<OpMAYBESIB_set> are intended to
be a low-level portable way to set C<op_sibling> / C<op_sibparent> while
also updating C<op_moresib>.  The first sets the sibling pointer to a new
sibling, the second makes the op the last sibling, and the third
conditionally does the first or second action.  Note that unlike
C<op_sibling_splice()> these macros won't maintain consistency in the
parent at the same time (e.g. by updating C<op_first> and C<op_last> where
appropriate).

A C-level C<Perl_op_parent()> function and a perl-level C<B::OP::parent()>
method have been added. The C function only exists under
C<-DPERL_OP_PARENT> builds (using it is build-time error on vanilla
perls).  C<B::OP::parent()> exists always, but on a vanilla build it
always returns C<NULL>. Under C<-DPERL_OP_PARENT>, they return the parent
of the current op, if any. The variable C<$B::OP::does_parent> allows you
to determine whether C<B> supports retrieving an op's parent.

XXX C<-DPERL_OP_PARENT> was introduced in 5.21.2, but the interface was
changed considerably in 5.21.11. If you updated your code before the
5.21.11 changes, it may require further revision. The main changes after
5.21.2 were:

=over 4

=item *

The C<OP_SIBLING> and C<OP_HAS_SIBLING> macros have been renamed
C<OpSIBLING> and C<OpHAS_SIBLING> for consistency with other
op-manipulating macros.

=item *

The C<op_lastsib> field has been renamed C<op_moresib>, and its meaning
inverted.

=item *

The macro C<OpSIBLING_set> has been removed, and has been superseded by
C<OpMORESIB_set> et al.

=item *

The C<op_sibling_splice()> function now accepts a null C<parent> argument
where the splicing doesn't affect the first or last ops in the sibling
chain

=back

=item *

Macros have been created to allow XS code to better manipulate the POSIX locale
category C<LC_NUMERIC>.  See L<perlapi/Locale-related functions and macros>.

=item *

The previous C<atoi> et al replacement function, C<grok_atou>, has now been
superseded by C<grok_atoUV>.  See L<perlclib> for details.

=item *

Added C<Perl_sv_get_backrefs()> to determine if an SV is a weak-referent.

Function either returns an SV * of type AV, which contains the set of
weakreferences which reference the passed in SV, or a simple RV * which
is the only weakref to this item.

=item *

The C<screaminstr> perl function has been removed. Although marked as
public API, it was undocumented and had no usage in CPAN modules. Calling
it has been fatal since 5.17.0.

=item *

C<newDEFSVOP>, C<block_start>, C<block_end> and C<intro_my> have been added
to the API.

=item *

The internal C<convert> function in F<op.c> has been renamed
C<op_convert_list> and added to the API.

=item *

C<sv_magic> no longer forbids "ext" magic on read-only values.  After all,
perl can't know whether the custom magic will modify the SV or not.
L<[perl #123103]|https://rt.perl.org/Ticket/Display.html?id=123103>.

=item *

Accessing L<perlapi/CvPADLIST> in an XSUB is now forbidden.
C<CvPADLIST> has been reused for a different internal purpose for XSUBs. Guard all
C<CvPADLIST> expressions with C<CvISXSUB()> if your code doesn't already block
XSUB CV*s from going through optree CV* expecting code.

=item *

SVs of type SVt_NV are now bodyless when a build configure and platform allow
it, specifically C<sizeof(NV) <= sizeof(IV)>. The bodyless trick is the same one
as for IVs since 5.9.2, but for NVs, unlike IVs, is not guaranteed on all
platforms and build configurations.

=item *

The C<$DB::single>, C<$DB::signal> and C<$DB::trace> now have set and
get magic that stores their values as IVs and those IVs are used when
testing their values in C<pp_dbstate>.  This prevents perl from
recursing infinitely if an overloaded object is assigned to any of those
variables.
L<[perl #122445]|https://rt.perl.org/Ticket/Display.html?id=122445>.

=item *

C<Perl_tmps_grow> which is marked as public API but is undocumented, has been
removed from public API. If you use C<EXTEND_MORTAL> macro in your XS code to
preextend the mortal stack, you are unaffected by this change.

=item *

XXX C<cv_name>, which was introduced in 5.21.4, has been changed incompatibly.
It now has a flags field that allows the caller to specify whether the name
should be fully qualified.  See L<perlapi/cv_name>.

=item *

Internally Perl no longer uses the C<SVs_PADMY> flag.  C<SvPADMY()> now
returns a true value for anything not marked PADTMP.  C<SVs_PADMY> is now
defined as 0.

=item *

The macros SETsv and SETsvUN have been removed. They were no longer used
in the core since commit 6f1401dc2a, and have not been found present on
CPAN.

=item *

The C<< SvFAKE >> bit (unused on HVs) got informally reserved by
David Mitchell for future work on vtables.

=item *

The C<sv_catpvn_flags> function accepts C<SV_CATBYTES> and C<SV_CATUTF8>
flags, which specify whether the appended string is bytes or utf8,
respectively.

=item *

A new opcode class, C<< METHOP >>, has been introduced. It holds
class/method related info needed at runtime to improve performance
of class/object method calls.

C<< OP_METHOD >> and C<< OP_METHOD_NAMED >> are moved from being
C<< UNOP/SVOP >> to being C<< METHOP >>.

=item *

C<save_re_context> no longer does anything and has been moved to F<mathoms.c>.

=item *

C<cv_name> is a new API function that can be passed a CV or GV.  It returns an
SV containing the name of the subroutine for use in diagnostics.
L<[perl #116735]|https://rt.perl.org/Ticket/Display.html?id=116735>
L<[perl #120441]|https://rt.perl.org/Ticket/Display.html?id=120441>

=item *

C<cv_set_call_checker_flags> is a new API function that works like
C<cv_set_call_checker>, except that it allows the caller to specify whether the
call checker requires a full GV for reporting the subroutine's name, or whether
it could be passed a CV instead.  Whatever value is passed will be acceptable
to C<cv_name>.  C<cv_set_call_checker> guarantees there will be a GV, but it
may have to create one on the fly, which is inefficient.
L<[perl #116735]|https://rt.perl.org/Ticket/Display.html?id=116735>

=item *

C<CvGV> (which is not part of the API) is now a more complex macro, which may
call a function and reify a GV.  For those cases where is has been used as a
boolean, C<CvHASGV> has been added, which will return true for CVs that
notionally have GVs, but without reifying the GV.  C<CvGV> also returns a GV
now for lexical subs.
L<[perl #120441]|https://rt.perl.org/Ticket/Display.html?id=120441>

=item *

Added L<perlapi/sync_locale>.  Changing the program's locale should be avoided
by XS code.  Nevertheless, certain non-Perl libraries called from XS, such as
C<Gtk> do so.  When this happens, Perl needs to be told that the locale has
changed.  Use this function to do so, before returning to Perl.

=item *

The defines and labels for the flags in the C<op_private> field of OPs are now
auto-generated from data in F<regen/op_private>.  The noticeable effect of this
is that some of the flag output of C<Concise> might differ slightly, and the
flag output of C<perl -Dx> may differ considerably (they both use the same set
of labels now).  Also in debugging builds, there is a new assert in
C<op_free()> that checks that the op doesn't have any unrecognized flags set in
C<op_private>.

=item *

The deprecated variable C<PL_sv_objcount> has been removed.

=item *

Perl now tries to keep the locale category C<LC_NUMERIC> set to "C"
except around operations that need it to be set to the program's
underlying locale.  This protects the many XS modules that cannot cope
with the decimal radix character not being a dot.  Prior to this
release, Perl initialized this category to "C", but a call to
C<POSIX::setlocale()> would change it.  Now such a call will change the
underlying locale of the C<LC_NUMERIC> category for the program, but the
locale exposed to XS code will remain "C".  There are new macros
to manipulate the LC_NUMERIC locale, including
C<STORE_LC_NUMERIC_SET_TO_NEEDED> and
C<STORE_LC_NUMERIC_FORCE_TO_UNDERLYING>.
See L<perlapi/Locale-related functions and macros>.

=item *

A new macro L<C<isUTF8_CHAR>|perlapi/isUTF8_CHAR> has been written which
efficiently determines if the string given by its parameters begins
with a well-formed UTF-8 encoded character.

=item *

The following private API functions had their context parameter removed,
C<Perl_cast_ulong>,  C<Perl_cast_i32>, C<Perl_cast_iv>,    C<Perl_cast_uv>,
C<Perl_cv_const_sv>, C<Perl_mg_find>,  C<Perl_mg_findext>, C<Perl_mg_magical>,
C<Perl_mini_mktime>, C<Perl_my_dirfd>, C<Perl_sv_backoff>, C<Perl_utf8_hop>.

Users of the public API prefix-less calls remain unaffected.

=item *

The PADNAME and PADNAMELIST types are now separate types, and no longer
simply aliases for SV and AV.
L<[perl #123223]|https://rt.perl.org/Ticket/Display.html?id=123223>.

=item *

Pad names are now always UTF8.  The C<PadnameUTF8> macro always returns
true.  Previously, this was effectively the case already, but any support
for two different internal representations of pad names has now been
removed.

=item *

A new op class, C<UNOP_AUX>, has been added. This is a subclass of
C<UNOP> with an C<op_aux> field added, which points to an array of unions
of C<UV>, C<SV*> etc. It is intended for where an op needs to store more data
than a simple C<op_sv> or whatever. Currently the only op of this type is
C<OP_MULTIDEREF> (see below).

=item *

A new op has been added, C<OP_MULTIDEREF>, which performs one or more
nested array and hash lookups where the key is a constant or simple
variable. For example the expression C<$a[0]{$k}[$i]>, which previously
involved ten C<rv2Xv>, C<Xelem>, C<gvsv> and C<const> ops is now performed
by a single C<multideref> op. It can also handle C<local>, C<exists> and
C<delete>. A non-simple index expression, such as C<[$i+1]> is still done
using C<aelem>/C<helem>, and single-level array lookup with a small constant
index is still done using C<aelemfast>.

=back

=head1 Selected Bug Fixes

=over 4

=item *

C<pack("D", $x)> and C<pack("F", $x)> now zero the padding on x86 long double
builds.  GCC 4.8 and later, under some build options, would either overwrite
the zero-initialized padding, or bypass the initialized buffer entirely.  This
caused F<op/pack.t> to fail.
L<[perl #123971]|https://rt.perl.org/Ticket/Display.html?id=123971>

=item *

Extending an array cloned from a parent thread could result in "Modification of
a read-only value attempted" errors when attempting to modify the new elements.
L<[perl #124127]|https://rt.perl.org/Ticket/Display.html?id=124127>

=item *

An assertion failure and subsequent crash with C<< *x=<y> >> has been fixed.
L<[perl #123790]|https://rt.perl.org/Ticket/Display.html?id=123790>

=item *

XXX An optimization for state variable initialization introduced in Perl 5.21.6 has
been reverted because it was found to exacerbate some other existing buggy
behaviour.
L<[perl #124160]|https://rt.perl.org/Ticket/Display.html?id=124160>

=item *

XXX The extension of another optimization to cover more ops in Perl 5.21 has also
been reverted to its Perl 5.20 state as a temporary fix for regression issues
that it caused.
L<[perl #123790]|https://rt.perl.org/Ticket/Display.html?id=123790>

=item *

A possible crashing/looping bug has been fixed.
L<[perl #124099]|https://rt.perl.org/Ticket/Display.html?id=124099>

=item *

UTF-8 variable names used in array indexes, unquoted UTF-8 HERE-document
terminators and UTF-8 function names all now work correctly.
L<[perl #124113]|https://rt.perl.org/Ticket/Display.html?id=124113>

=item *

Repeated global pattern matches in scalar context on large tainted strings were
exponentially slow depending on the current match position in the string.
L<[perl #123202]|https://rt.perl.org/Ticket/Display.html?id=123202>

=item *

Various crashes due to the parser getting confused by syntax errors have been
fixed.
L<[perl #123801]|https://rt.perl.org/Ticket/Display.html?id=123801>
L<[perl #123802]|https://rt.perl.org/Ticket/Display.html?id=123802>
L<[perl #123955]|https://rt.perl.org/Ticket/Display.html?id=123955>
L<[perl #123995]|https://rt.perl.org/Ticket/Display.html?id=123995>

=item *

C<split> in the scope of lexical C<$>_ has been fixed not to fail assertions.
L<[perl #123763]|https://rt.perl.org/Ticket/Display.html?id=123763>

=item *

C<my $x : attr> syntax inside various list operators no longer fails
assertions.
L<[perl #123817]|https://rt.perl.org/Ticket/Display.html?id=123817>

=item *

An C<@> sign in quotes followed by a non-ASCII digit (which is not a valid
identifier) would cause the parser to crash, instead of simply trying the C<@> as
literal.  This has been fixed.
L<[perl #123963]|https://rt.perl.org/Ticket/Display.html?id=123963>

=item *

C<*bar::=*foo::=*glob_with_hash> has been crashing since Perl 5.14, but no
longer does.
L<[perl #123847]|https://rt.perl.org/Ticket/Display.html?id=123847>

=item *

C<foreach> in scalar context was not pushing an item on to the stack, resulting
in bugs.  (S<C<print 4, scalar do { foreach(@x){} } + 1>> would print 5.)  It has
been fixed to return C<undef>.
L<[perl #124004]|https://rt.perl.org/Ticket/Display.html?id=124004>

=item *

A regression in the behaviour of the C<readline> built-in function, caused by
the introduction of the C<< <<>> >> operator, has been fixed.
L<[perl #123990]|https://rt.perl.org/Ticket/Display.html?id=123990>

=item *

Several cases of data used to store environment variable contents in core C
code being potentially overwritten before being used have been fixed.
L<[perl #123748]|https://rt.perl.org/Ticket/Display.html?id=123748>

=item *

Patterns starting with C</.*/> are now fast again.
L<[perl #123743]|https://rt.perl.org/Ticket/Display.html?id=123743>.

=item *

The original visible value of C<$/> is now preserved when it is set to
an invalid value.  Previously if you set C<$/> to a reference to an
array, for example, perl would produce a runtime error and not set
C<PL_rs>, but perl code that checked C<$/> would see the array
reference.
L<[perl #123218]|https://rt.perl.org/Ticket/Display.html?id=123218>.

=item *

In a regular expression pattern, a POSIX class, like C<[:ascii:]>, must
be inside a bracketed character class, like C<qr/[[:ascii:]]/>.  A
warning is issued when something looking like a POSIX class is not
inside a bracketed class.  That warning wasn't getting generated when
the POSIX class was negated: C<[:^ascii:]>.  This is now fixed.

=item *

Fix a couple of size calculation overflows.
L<[perl #123554]|https://rt.perl.org/Ticket/Display.html?id=123554>.

=item *

Perl 5.14.0 introduced a bug whereby C<eval { LABEL: }> would crash.  This
has been fixed.
L<[perl #123652]|https://rt.perl.org/Ticket/Display.html?id=123652>.

=item *

Various crashes due to the parser getting confused by syntax errors have
been fixed.
L<[perl #123617]|https://rt.perl.org/Ticket/Display.html?id=123617>.
L<[perl #123737]|https://rt.perl.org/Ticket/Display.html?id=123737>.
L<[perl #123753]|https://rt.perl.org/Ticket/Display.html?id=123753>.
L<[perl #123677]|https://rt.perl.org/Ticket/Display.html?id=123677>.

=item *

Code like C</$a[/> used to read the next line of input and treat it as
though it came immediately after the opening bracket.  Some invalid code
consequently would parse and run, but some code caused crashes, so this is
now disallowed.
L<[perl #123712]|https://rt.perl.org/Ticket/Display.html?id=123712>.

=item *

Fix argument underflow for C<pack>.
L<[perl #123874]|https://rt.perl.org/Ticket/Display.html?id=123874>.

=item *

Fix handling of non-strict C<\x{}>. Now C<\x{}> is equivalent to C<\x{0}>
instead of faulting.

=item *

C<stat -t> is now no longer treated as stackable, just like C<-t stat>.
L<[perl #123816]|https://rt.perl.org/Ticket/Display.html?id=123816>.

=item *

The following no longer causes a SEGV: C<qr{x+(y(?0))*}>.

=item *

Fixed infinite loop in parsing backrefs in regexp patterns.

=item *

Several minor bug fixes in behavior of Inf and NaN, including
warnings when stringifying Inf-like or NaN-like strings. For example,
"NaNcy" doesn't numify to NaN anymore.

=item *

Only stringy classnames are now shared. This fixes some failures in L<autobox>.
L<[perl #100819]|https://rt.cpan.org/Ticket/Display.html?id=100819>.

=item *

A bug in regular expression patterns that could lead to segfaults and
other crashes has been fixed.  This occurred only in patterns compiled
with C<"/i">, while taking into account the current POSIX locale (this usually
means they have to be compiled within the scope of C<S<"use locale">>),
and there must be a string of at least 128 consecutive bytes to match.
L<[perl #123539]|https://rt.perl.org/Ticket/Display.html?id=123539>.

=item *

C<s///> now works on very long strings instead of dying with 'Substitution
loop'.
L<[perl #103260]|https://rt.perl.org/Ticket/Display.html?id=103260>.
L<[perl #123071]|https://rt.perl.org/Ticket/Display.html?id=123071>.

=item *

C<gmtime> no longer crashes with not-a-number values.
L<[perl #123495]|https://rt.perl.org/Ticket/Display.html?id=123495>.

=item *

C<\()> (reference to an empty list) and C<y///> with lexical C<$_> in scope
could do a bad write past the end of the stack.  They have been fixed
to extend the stack first.

=item *

C<prototype()> with no arguments used to read the previous item on the
stack, so C<print "foo", prototype()> would print foo's prototype.  It has
been fixed to infer C<$_> instead.
L<[perl #123514]|https://rt.perl.org/Ticket/Display.html?id=123514>.

=item *

Some cases of lexical state subs inside predeclared subs could crash but no
longer do.

=item *

Some cases of nested lexical state subs inside anonymous subs could cause
'Bizarre copy' errors or possibly even crash.

=item *

When trying to emit warnings, perl's default debugger (F<perl5db.pl>) was
sometimes giving 'Undefined subroutine &DB::db_warn called' instead.  This
bug, which started to occur in Perl 5.18, has been fixed.
L<[perl #123553]|https://rt.perl.org/Ticket/Display.html?id=123553>.

=item *

Certain syntax errors in substitutions, such as C<< s/${E<lt>E<gt>{})// >>, would
crash, and had done so since Perl 5.10.  (In some cases the crash did not
start happening till 5.16.)  The crash has, of course, been fixed.
L<[perl #123542]|https://rt.perl.org/Ticket/Display.html?id=123542>.

=item *

A repeat expression like C<33 x ~3> could cause a large buffer
overflow since the new output buffer size was not correctly handled by
SvGROW().  An expression like this now properly produces a memory wrap
panic.
L<[perl #123554]|https://rt.perl.org/Ticket/Display.html?id=123554>.

=item *

C<< formline("@...", "a"); >> would crash.  The C<FF_CHECKNL> case in
pp_formline() didn't set the pointer used to mark the chop position,
which led to the C<FF_MORE> case crashing with a segmentation fault.
This has been fixed.
L<[perl #123538]|https://rt.perl.org/Ticket/Display.html?id=123538>.

=item *

A possible buffer overrun and crash when parsing a literal pattern during
regular expression compilation has been fixed.
L<[perl #123604]|https://rt.perl.org/Ticket/Display.html?id=123604>.

=item *

C<fchmod()> and C<futimes()> now set C<$!> when they fail due to being
passed a closed file handle.
L<[perl #122703]|https://rt.perl.org/Ticket/Display.html?id=122703>.

=item *

op_free() no longer crashes due to a stack overflow when freeing a
deeply recursive op tree.
L<[perl #108276]|https://rt.perl.org/Ticket/Display.html?id=108276>.

=item *

scalarvoid() would crash due to a stack overflow when processing a
deeply recursive op tree.
L<[perl #108276]|https://rt.perl.org/Ticket/Display.html?id=108276>.

=item *

In Perl 5.20.0, C<$^N> accidentally had the internal UTF8 flag turned off
if accessed from a code block within a regular expression, effectively
UTF8-encoding the value.  This has been fixed.
L<[perl #123135]|https://rt.perl.org/Ticket/Display.html?id=123135>.

=item *

A failed C<semctl> call no longer overwrites existing items on the stack,
causing C<(semctl(-1,0,0,0))[0]> to give an "uninitialized" warning.

=item *

C<else{foo()}> with no space before C<foo> is now better at assigning the
right line number to that statement.
L<[perl #122695]|https://rt.perl.org/Ticket/Display.html?id=122695>.

=item *

Sometimes the assignment in C<@array = split> gets optimised and C<split>
itself writes directly to the array.  This caused a bug, preventing this
assignment from being used in lvalue context.  So
C<(@a=split//,"foo")=bar()> was an error.  (This bug probably goes back to
Perl 3, when the optimisation was added.)  This optimisation, and the bug,
started to happen in more cases in XXX 5.21.5.  It has now been fixed.
L<[perl #123057]|https://rt.perl.org/Ticket/Display.html?id=123057>.

=item *

When argument lists that fail the checks installed by subroutine
signatures, the resulting error messages now give the file and line number
of the caller, not of the called subroutine.
L<[perl #121374]|https://rt.perl.org/Ticket/Display.html?id=121374>.

=item *

Flip-flop operators (C<..> and C<...> in scalar context) used to maintain
a separate state for each recursion level (the number of times the
enclosing sub was called recursively), contrary to the documentation.  Now
each closure has one internal state for each flip-flop.
L<[perl #122829]|https://rt.perl.org/Ticket/Display.html?id=122829>.

=item *

C<use>, C<no>, statement labels, special blocks (C<BEGIN>) and pod are now
permitted as the first thing in a C<map> or C<grep> block, the block after
C<print> or C<say> (or other functions) returning a handle, and within
C<${...}>, C<@{...}>, etc.
L<[perl #122782]|https://rt.perl.org/Ticket/Display.html?id=122782>.

=item *

The repetition operator C<x> now propagates lvalue context to its left-hand
argument when used in contexts like C<foreach>.  That allows
S<C<for(($#that_array)x2) { ... }>> to work as expected if the loop modifies
$_.

=item *

C<(...) x ...> in scalar context used to corrupt the stack if one operand
were an object with "x" overloading, causing erratic behaviour.
L<[perl #121827]|https://rt.perl.org/Ticket/Display.html?id=121827>.

=item *

Assignment to a lexical scalar is often optimised away (as mentioned under
L</Performance Enhancements>).  Various bugs related to this optimisation
have been fixed.  Certain operators on the right-hand side would sometimes
fail to assign the value at all or assign the wrong value, or would call
STORE twice or not at all on tied variables.  The operators affected were
C<$foo++>, C<$foo-->, and C<-$foo> under C<use integer>, C<chomp>, C<chr>
and C<setpgrp>.

=item *

List assignments were sometimes buggy if the same scalar ended up on both
sides of the assignment due to used of C<tied>, C<values> or C<each>.  The
result would be the wrong value getting assigned.

=item *

C<setpgrp($nonzero)> (with one argument) was accidentally changed in 5.16
to mean C<setpgrp(0)>.  This has been fixed.

=item *

C<__SUB__> could return the wrong value or even corrupt memory under the
debugger (the C<-d> switch) and in subs containing C<eval $string>.

=item *

When S<C<sub () { $var }>> becomes inlinable, it now returns a different
scalar each time, just as a non-inlinable sub would, though Perl still
optimises the copy away in cases where it would make no observable
difference.

=item *

S<C<my sub f () { $var }>> and S<C<sub () : attr { $var }>> are no longer
eligible for inlining.  The former would crash; the latter would just
throw the attributes away.  An exception is made for the little-known
":method" attribute, which does nothing much.

=item *

Inlining of subs with an empty prototype is now more consistent than
before.  Previously, a sub with multiple statements, all but the last
optimised away, would be inlinable only if it were an anonymous sub
containing a string C<eval> or C<state> declaration or closing over an
outer lexical variable (or any anonymous sub under the debugger).  Now any
sub that gets folded to a single constant after statements have been
optimised away is eligible for inlining.  This applies to things like C<sub
() { jabber() if DEBUG; 42 }>.

Some subroutines with an explicit C<return> were being made inlinable,
contrary to the documentation,  Now C<return> always prevents inlining.

=item *

On some systems, such as VMS, C<crypt> can return a non-ASCII string.  If a
scalar assigned to had contained a UTF8 string previously, then C<crypt>
would not turn off the UTF8 flag, thus corrupting the return value.  This
would happen with C<$lexical = crypt ...>.

=item *

C<crypt> no longer calls C<FETCH> twice on a tied first argument.

=item *

An unterminated here-doc on the last line of a quote-like operator
(C<qq[${ <<END }]>, C</(?{ <<END })/>) no longer causes a double free.  It
started doing so in 5.18.

=item *

Fixed two assertion failures introduced into C<-DPERL_OP_PARENT>
builds.
L<[perl #108276]|https://rt.perl.org/Ticket/Display.html?id=108276>.

=item *

C<index()> and C<rindex()> no longer crash when used on strings over 2GB in
size.
L<[perl #121562]|https://rt.perl.org/Ticket/Display.html?id=121562>.

=item *

A small previously intentional memory leak in PERL_SYS_INIT/PERL_SYS_INIT3 on
Win32 builds was fixed. This might affect embedders who repeatedly create and
destroy perl engines within the same process.

=item *

C<POSIX::localeconv()> now returns the data for the program's underlying
locale even when called from outside the scope of S<C<use locale>>.

=item *

C<POSIX::localeconv()> now works properly on platforms which don't have
C<LC_NUMERIC> and/or C<LC_MONETARY>, or for which Perl has been compiled
to disregard either or both of these locale categories.  In such
circumstances, there are now no entries for the corresponding values in
the hash returned by C<localeconv()>.

=item *

C<POSIX::localeconv()> now marks appropriately the values it returns as
UTF-8 or not.  Previously they were always returned as bytes, even if
they were supposed to be encoded as UTF-8.

=item *

On Microsoft Windows, within the scope of C<S<use locale>>, the following
POSIX character classes gave results for many locales that did not
conform to the POSIX standard:
C<[[:alnum:]]>,
C<[[:alpha:]]>,
C<[[:blank:]]>,
C<[[:digit:]]>,
C<[[:graph:]]>,
C<[[:lower:]]>,
C<[[:print:]]>,
C<[[:punct:]]>,
C<[[:upper:]]>,
C<[[:word:]]>,
and
C<[[:xdigit:]]>.
This was because the underlying Microsoft implementation does not
follow the standard.  Perl now takes special precautions to correct for
this.

=item *

Many issues have been detected by L<Coverity|http://www.coverity.com/> and
fixed.

=item *

system() and friends should now work properly on more Android builds.

Due to an oversight, the value specified through C<-Dtargetsh> to F<Configure>
would end up being ignored by some of the build process.  This caused perls
cross-compiled for Android to end up with defective versions of C<system()>,
exec() and backticks: the commands would end up looking for C</bin/sh>
instead of C</system/bin/sh>, and so would fail for the vast majority
of devices, leaving C<$!> as C<ENOENT>.

=item *

C<qr(...\(...\)...)>,
C<qr[...\[...\]...]>,
and
C<qr{...\{...\}...}>
now work.  Previously it was impossible to escape these three
left-characters with a backslash within a regular expression pattern
where otherwise they would be considered metacharacters, and the pattern
opening delimiter was the character, and the closing delimiter was its
mirror character.

=item *

C<< s///e >> on tainted utf8 strings got C<< pos() >> messed up. This bug,
introduced in 5.20, is now fixed.
L<[perl #122148]|https://rt.perl.org/Ticket/Display.html?id=122148>.

=item *

A non-word boundary in a regular expression (C<< \B >>) did not always
match the end of the string; in particular C<< q{} =~ /\B/ >> did not
match. This bug, introduced in perl 5.14, is now fixed.
L<[perl #122090]|https://rt.perl.org/Ticket/Display.html?id=122090>.

=item *

C<< " P" =~ /(?=.*P)P/ >> should match, but did not. This is now fixed.
L<[perl #122171]|https://rt.perl.org/Ticket/Display.html?id=122171>.

=item *

Failing to compile C<use Foo> in an eval could leave a spurious
C<BEGIN> subroutine definition, which would produce a "Subroutine
BEGIN redefined" warning on the next use of C<use>, or other C<BEGIN>
block.
L<[perl #122107]|https://rt.perl.org/Ticket/Display.html?id=122107>.

=item *

C<method { BLOCK } ARGS> syntax now correctly parses the arguments if they
begin with an opening brace.
L<[perl #46947]|https://rt.perl.org/Ticket/Display.html?id=46947>.

=item *

External libraries and Perl may have different ideas of what the locale is.
This is problematic when parsing version strings if the locale's numeric
separator has been changed.  Version parsing has been patched to ensure
it handles the locales correctly.
L<[perl #121930]|https://rt.perl.org/Ticket/Display.html?id=121930>.

=item *

A bug has been fixed where zero-length assertions and code blocks inside of a
regex could cause C<pos> to see an incorrect value.
L<[perl #122460]|https://rt.perl.org/Ticket/Display.html?id=122460>.

=item *

Constant dereferencing now works correctly for typeglob constants.  Previously
the glob was stringified and its name looked up.  Now the glob itself is used.
L<[perl #69456]|https://rt.perl.org/Ticket/Display.html?id=69456>

=item *

When parsing a funny character (C<$> C<@> C<%> C<&)> followed by braces,
the parser no
longer tries to guess whether it is a block or a hash constructor (causing a
syntax error when it guesses the latter), since it can only be a block.

=item *

S<C<undef $reference>> now frees the referent immediately, instead of hanging on
to it until the next statement.
L<[perl #122556]|https://rt.perl.org/Ticket/Display.html?id=122556>

=item *

Various cases where the name of a sub is used (autoload, overloading, error
messages) used to crash for lexical subs, but have been fixed.

=item *

Bareword lookup now tries to avoid vivifying packages if it turns out the
bareword is not going to be a subroutine name.

=item *

Compilation of anonymous constants (e.g., C<sub () { 3 }>) no longer deletes
any subroutine named C<__ANON__> in the current package.  Not only was
C<*__ANON__{CODE}> cleared, but there was a memory leak, too.  This bug goes
back to Perl 5.8.0.

=item *

Stub declarations like C<sub f;> and C<sub f ();> no longer wipe out constants
of the same name declared by C<use constant>.  This bug was introduced in Perl
5.10.0.

=item *

C<qr/[\N{named sequence}]/> now works properly in many instances.  Some names
known to C<\N{...}> refer to a sequence of multiple characters, instead of the
usual single character.  Bracketed character classes generally only match
single characters, but now special handling has been added so that they can
match named sequences, but not if the class is inverted or the sequence is
specified as the beginning or end of a range.  In these cases, the only
behavior change from before is a slight rewording of the fatal error message
given when this class is part of a C<?[...])> construct.  When the C<[...]>
stands alone, the same non-fatal warning as before is raised, and only the
first character in the sequence is used, again just as before.

=item *

Tainted constants evaluated at compile time no longer cause unrelated
statements to become tainted.
L<[perl #122669]|https://rt.perl.org/Ticket/Display.html?id=122669>

=item *

S<C<open $$fh, ...>>, which vivifies a handle with a name like C<"main::_GEN_0">, was
not giving the handle the right reference count, so a double free could happen.

=item *

When deciding that a bareword was a method name, the parser would get confused
if an C<our> sub with the same name existed, and look up the method in the
package of the C<our> sub, instead of the package of the invocant.

=item *

The parser no longer gets confused by C<\U=> within a double-quoted string.  It
used to produce a syntax error, but now compiles it correctly.
L<[perl #80368]|https://rt.perl.org/Ticket/Display.html?id=80368>

=item *

It has always been the intention for the C<-B> and C<-T> file test operators to
treat UTF-8 encoded files as text.  (L<perlfunc|perlfunc/-X FILEHANDLE> has
been updated to say this.)  Previously, it was possible for some files to be
considered UTF-8 that actually weren't valid UTF-8.  This is now fixed.  The
operators now work on EBCDIC platforms as well.

=item *

Under some conditions warning messages raised during regular expression pattern
compilation were being output more than once.  This has now been fixed.

=item *

A regression has been fixed that was introduced in Perl 5.20.0 (fixed in Perl
5.20.1 as well as here) in which a UTF-8 encoded regular expression pattern
that contains a single ASCII lowercase letter does not match its uppercase
counterpart.
L<[perl #122655]|https://rt.perl.org/Ticket/Display.html?id=122655>

=item *

Constant folding could incorrectly suppress warnings if lexical warnings (C<use
warnings> or C<no warnings>) were not in effect and C<$^W> were false at
compile time and true at run time.

=item *

Loading UTF8 tables during a regular expression match could cause assertion
failures under debugging builds if the previous match used the very same
regular expression.
L<[perl #122747]|https://rt.perl.org/Ticket/Display.html?id=122747>

=item *

Thread cloning used to work incorrectly for lexical subs, possibly causing
crashes or double frees on exit.

=item *

Since Perl 5.14.0, deleting C<$SomePackage::{__ANON__}> and then undefining an
anonymous subroutine could corrupt things internally, resulting in
L<Devel::Peek> crashing or L<B.pm|B> giving nonsensical data.  This has been
fixed.

=item *

S<C<(caller $n)[3]>> now reports names of lexical subs, instead of treating them
as C<"(unknown)">.

=item *

C<sort subname LIST> now supports lexical subs for the comparison routine.

=item *

Aliasing (e.g., via C<*x = *y>) could confuse list assignments that mention the
two names for the same variable on either side, causing wrong values to be
assigned.
L<[perl #15667]|https://rt.perl.org/Ticket/Display.html?id=15667>

=item *

Long here-doc terminators could cause a bad read on short lines of input.  This
has been fixed.  It is doubtful that any crash could have occurred.  This bug
goes back to when here-docs were introduced in Perl 3.000 twenty-five years
ago.

=item *

An optimization in C<split> to treat C<split/^/> like C<split/^/m> had the
unfortunate side-effect of also treating C<split/\A/> like C<split/^/m>, which
it should not.  This has been fixed.  (Note, however, that C<split/^x/> does
not behave like C<split/^x/m>, which is also considered to be a bug and will be
fixed in a future version.)
L<[perl #122761]|https://rt.perl.org/Ticket/Display.html?id=122761>

=item *

The little-known S<C<my Class $var>> syntax (see L<fields> and L<attributes>)
could get confused in the scope of C<use utf8> if C<Class> were a constant
whose value contained Latin-1 characters.

=item *

Locking and unlocking values via L<Hash::Util> or C<Internals::SvREADONLY>
no longer has any effect on values that are read-only to begin.
Previously, unlocking such values could result in crashes, hangs or
other erratic behaviour.

=item *

The flip-flop operator (C<..> in scalar context) would return the same
scalar each time, unless the containing subroutine was called recursively.
Now it always returns a new scalar.
L<[perl #122829]|https://rt.perl.org/Ticket/Display.html?id=122829>.

=item *

Some unterminated C<(?(...)...)> constructs in regular expressions would
either crash or give erroneous error messages.  C</(?(1)/> is one such
example.

=item *

S<C<pack "w", $tied>> no longer calls FETCH twice.

=item *

List assignments like S<C<($x, $z) = (1, $y)>> now work correctly if C<$x> and
C<$y> have been aliased by C<foreach>.

=item *

Some patterns including code blocks with syntax errors, such as
C</ (?{(^{})/>, would hang or fail assertions on debugging builds.  Now
they produce errors.

=item *

An assertion failure when parsing C<sort> with debugging enabled has been
fixed.
L<[perl #122771]|https://rt.perl.org/Ticket/Display.html?id=122771>.

=item *

S<C<*a = *b; @a = split //, $b[1]>> could do a bad read and produce junk
results.

=item *

In S<C<() = @array = split>>, the S<C<() =>> at the beginning no longer confuses
the optimizer, making it assume a limit of 1.

=item *

Fatal warnings no longer prevent the output of syntax errors.
L<[perl #122966]|https://rt.perl.org/Ticket/Display.html?id=122966>.

=item *

Fixed a NaN double to long double conversion error on VMS. For quiet NaNs
(and only on Itanium, not Alpha) negative infinity instead of NaN was
produced.

=item *

Fixed the issue that caused C<< make distclean >> to leave files behind
that shouldn't.
L<[perl #122820]|https://rt.perl.org/Ticket/Display.html?id=122820>.

=item *

AIX now sets the length in C<< getsockopt >> correctly.
L<[perl #120835]|https://rt.perl.org/Ticket/Display.html?id=120835>.
L<[cpan #91183]|https://rt.cpan.org/Ticket/Display.html?id=91183>.
L<[cpan #85570]|https://rt.cpan.org/Ticket/Display.html?id=85570>.

=item *

During the pattern optimization phase, we no longer recurse into
C<GOSUB>/C<GOSTART> when not C<SCF_DO_SUBSTR>. This prevents the optimizer
to run "forever" and exhaust all memory.
L<[perl #122283]|https://rt.perl.org/Ticket/Display.html?id=122283>.

=item *

F<< t/op/crypt.t >> now performs SHA-256 algorithm if the default one
is disabled.
L<[perl #121591]|https://rt.perl.org/Ticket/Display.html?id=121591>.

=item *

Fixed an off-by-one error when setting the size of shared array.
L<[perl #122950]|https://rt.perl.org/Ticket/Display.html?id=122950>.

=item *

Fixed a bug that could cause perl to execute an infinite loop during
compilation.
L<[perl #122995]|https://rt.perl.org/Ticket/Display.html?id=122995>.

=item *

On Win32, restoring in a child pseudo-process a variable that was
C<local()>ed in a parent pseudo-process before the C<fork> happened caused
memory corruption and a crash in the child pseudo-process (and therefore OS
process).
L<[perl #40565]|https://rt.perl.org/Ticket/Display.html?id=40565>.

=item *

Calling C<write> on a format with a C<^**> field could produce a panic
in C<sv_chop()> if there were insufficient arguments or if the variable
used to fill the field was empty.
L<[perl #123245]|https://rt.perl.org/Ticket/Display.html?id=123245>.

=item *

Non-ASCII lexical sub names (use in error messages) on longer have extra
junk on the end.

=item *

The C<\@> subroutine prototype no longer flattens parenthesized arrays
(taking a reference to each element), but takes a reference to the array
itself.
L<[perl #47363]|https://rt.perl.org/Ticket/Display.html?id=47363>.

=item *

A block containing nothing except a C-style C<for> loop could corrupt the
stack, causing lists outside the block to lose elements or have elements
overwritten.  This could happen with C<map { for(...){...} } ...> and with
lists containing C<do { for(...){...} }>.
L<[perl #123286]|https://rt.perl.org/Ticket/Display.html?id=123286>.

=item *

C<scalar()> now propagates lvalue context, so that
S<C<for(scalar($#foo)) { ... }>> can modify C<$#foo> through C<$_>.

=item *

C<qr/@array(?{block})/> no longer dies with "Bizarre copy of ARRAY".
L<[perl #123344]|https://rt.perl.org/Ticket/Display.html?id=123344>.

=item *

S<C<eval '$variable'>> in nested named subroutines would sometimes look up a
global variable even with a lexical variable in scope.

=item *

In perl 5.20.0, C<sort CORE::fake> where 'fake' is anything other than a
keyword started chopping of the last 6 characters and treating the result
as a sort sub name.  The previous behaviour of treating "CORE::fake" as a
sort sub name has been restored.
L<[perl #123410]|https://rt.perl.org/Ticket/Display.html?id=123410>.

=item *

Outside of C<use utf8>, a single-character Latin-1 lexical variable is
disallowed.  The error message for it, "Can't use global C<$foo>...", was
giving garbage instead of the variable name.

=item *

C<readline> on a nonexistent handle was causing C<${^LAST_FH}> to produce a
reference to an undefined scalar (or fail an assertion).  Now
C<${^LAST_FH}> ends up undefined.

=item *

C<(...)x...> in void context now applies scalar context to the left-hand
argument, instead of the context the current sub was called in.
L<[perl #123020]|https://rt.perl.org/Ticket/Display.html?id=123020>.

=back

=head1 Known Problems

=over 4

=item *

A goal is for Perl to be able to be recompiled to work reasonably well on any
Unicode version.  In Perl 5.22, though, the earliest such version is Unicode
5.1 (current is 7.0).

=item *

EBCDIC platforms

=over 4

=item *

The C<cmp> (and hence C<sort>) operators do not necessarily give the
correct results when both operands are UTF-EBCDIC encoded strings and
there is a mixture of ASCII and/or control characters, along with other
characters.

=item *

Ranges containing C<\N{...}> in the C<tr///> (and C<y///>)
transliteration operators are treated differently than the equivalent
ranges in regular expression pattersn.  They should, but don't, cause
the values in the ranges to all be treated as Unicode code points, and
not native ones.  (L<perlre/Version 8 Regular Expressions> gives
details as to how it should work.)

=item *

Encode and encoding are mostly broken.

=item *

Many CPAN modules that are shipped with core show failing tests.

=item *

C<pack>/C<unpack> with C<"U0"> format may not work properly.

=back

=item *

The following modules are known to have test failures with this version of
Perl.  Patches have been submitted, so there will hopefully be new releases
soon:

=over

=item *

L<B::Generate> version 1.50

=item *

L<B::Utils> version 0.25

=item *

L<Dancer> version 1.3130

=item *

L<Data::Alias> version 1.18

=item *

L<Data::Util> version 0.63

=item *

L<Devel::Spy> version 0.07

=item *

L<invoker> version 0.34

=item *

L<Lexical::Var> version 0.009

=item *

L<Mason> version 2.22

=item *

L<NgxQueue> version 0.02

=item *

L<Padre> version 1.00

=item *

L<Parse::Keyword> 0.08

=back

=back

=head1 Obituary

Brian McCauley died on May 8, 2015.  He was a frequent poster to Usenet, Perl
Monks, and other Perl forums, and made several CPAN contributions under the
nick NOBULL, including to the Perl FAQ.  He attended almost every
YAPC::Europe, and indeed, helped organise YAPC::Europe 2006 and the QA
Hackathon 2009.  His wit and his delight in intricate systems were
particularly apparent in his love of board games; many Perl mongers will
have fond memories of playing Fluxx and other games with Brian.  He will be
missed.

=head1 Acknowledgements

XXX Generate this with:

  perl Porting/acknowledgements.pl v5.20.0..HEAD

=head1 Reporting Bugs

If you find what you think is a bug, you might check the articles recently
posted to the comp.lang.perl.misc newsgroup and the perl bug database at
https://rt.perl.org/ .  There may also be information at
http://www.perl.org/ , the Perl Home Page.

If you believe you have an unreported bug, please run the L<perlbug> program
included with your release.  Be sure to trim your bug down to a tiny but
sufficient test case.  Your bug report, along with the output of C<perl -V>,
will be sent off to perlbug@perl.org to be analysed by the Perl porting team.

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

=head1 SEE ALSO

The F<Changes> file for an explanation of how to view exhaustive details on
what changed.

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

The F<README> file for general stuff.

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

=cut