perl5360delta: clarify future of use VERSION and strict

[perl5.git] / pod / perldelta.pod

=encoding utf8

=head1 NAME

perldelta - what is new for perl v5.36.0

=head1 DESCRIPTION

This document describes differences between the 5.34.0 release and the 5.36.0
release.

=head1 Core Enhancements

=head2 C<use v5.36>

As always, C<use v5.36> turns on the feature bundle for that version of Perl.

The 5.36 bundle enables the C<signatures> feature.  Introduced in Perl version
5.20.0, and modified several times since, the subroutine signatures feature is
now no longer considered experimental. It is now considered a stable language
feature and no longer prints a warning.

    use v5.36;

    sub add ($x, $y) {
      return $x + $y;
    }

Despite this, certain elements of signatured subroutines remain experimental;
see below.

The 5.36 bundle enables the C<isa> feature.  Introduced in Perl version 5.32.0,
this operator has remained unchanged since then. The operator is now considered
a stable language feature.  For more detail see L<perlop/Class Instance
Operator>.

The 5.36 bundle also I<disables> the features C<indirect>, and
C<multidimensional>.  These will forbid, respectively: the use of "indirect"
method calls (like C<$x = new Class;>); the use of a list expression as a hash
key to simulate sparse multidimensional arrays.  The specifics of these changes
can be found in L<feature>, but the short version is: this is a bit like having
more C<use strict> turned on, disabling features that cause more trouble than
they're worth.

Furthermore, C<use v5.36> will also enable warnings as if you'd written C<use
warnings>.

Finally, with this release, the experimental C<switch> feature, present in
every feature bundle since they were introduced in v5.10, has been removed from
the v5.36 bundle.  If you want to use it (against our advice), you'll have to
enable it explicitly.

=head2 -g command-line flag

A new command-line flag, -g, is available. It is a simpler alias for -0777.

For more information, see L<perlrun/-g>.

=head2 Unicode 14.0 is supported

See L<https://www.unicode.org/versions/Unicode14.0.0/> for details.

=head2 regex sets are no longer considered experimental

Prior to this release, the regex sets feature (officially named
"Extended Bracketed Character Classes") was considered experimental.
Introduced in Perl version 5.18.0, and modified several times since,
this is now considered a stable language feature and its use no longer
prints a warning.  See L<perlrecharclass/Extended Bracketed Character
Classes>.

=head2 Variable length lookbehind is mostly no longer considered experimental

Prior to this release, any form of variable length lookbehind was
considered experimental. With this release the experimental status has
been reduced to cover only lookbehind that contains capturing parenthesis.
This is because it is not clear if

    "aaz"=~/(?=z)(?<=(a|aa))/

should match and leave $1 equaling "a" or "aa". Currently it will match
the longest possible alternative, "aa". While we are confident that the overall
construct will now match only when it should, we are not confident that we
will keep the current "longest match" behavior.

=head2 SIGFPE no longer deferred

Floating-point exceptions are now delivered immediately, in the same way
as other "fault"-like signals such as SIGSEGV. This means one has at
least a chance to catch such a signal with a C<$SIG{FPE}> handler, e.g.
so that C<die> can report the line in perl that triggered it.

=head2 Stable boolean tracking

The "true" and "false" boolean values, often accessed by constructions like
C<!!0> and C<!!1>, as well as being returned from many core functions and
operators, now remember their boolean nature even through assignment into
variables. The new function C<is_bool()> in L<builtin> can check whether
a value has boolean nature.

This is likely to be useful when interoperating with other languages or
data-type serialisation, among other places.

=head2 iterating over multiple values at a time (experimental)

You can now iterate over multiple values at a time by specifying a list of
lexicals within parentheses. For example,

    for my ($key, $value) (%hash) { ... }
    for my ($left, $right, $gripping) (@moties) { ... }

Prior to perl v5.36, attempting to specify a list after C<for my> was a syntax
error.

This feature is currently experimental and will cause a warning of category
C<experimental::for_list>.  For more detail see L<perlsyn/Compound Statements>.
See also L</builtin::indexed> in this document, which is a handy companion to
n-at-a-time foreach.

=head2 builtin functions (experimental)

A new core module L<builtin> has been added, which provides documentation for
new always-present functions that are built into the interpreter.

    say "Reference type of arrays is ", builtin::reftype([]);

It also provides a lexical import mechanism for providing short name versions
of these functions.

    use builtin 'reftype';
    say "Reference type of arrays is ", reftype([]);

This builtin function mechanism and the functions it provides are all
currently B<experimental>.  We expect that C<builtin> itself will cease to be
experimental in the near future, but that individual functions in it may become
stable on an ongoing basis.  Other functions will be added to C<builtin> over
time.

For details, see L<builtin>, but here's a summary of builtin functions in
v5.36:

=over 4

=item builtin::trim

This function treats its argument as a string, returning the result of removing
all white space at its beginning and ending.

=item builtin::indexed

This function returns a list twice as big as its argument list, where each item
is preceded by its index within that list. This is primarily useful for using
the new C<foreach> syntax with multiple iterator variables to iterate over an
array or list, while also tracking the index of each item:

    use builtin 'indexed';

    foreach my ($index, $val) (indexed @array) {
        ...
    }

=item builtin::true, builtin::false, builtin::is_bool

C<true> and C<false> return boolean true and false values.  Perl is still perl,
and doesn't have strict typing of booleans, but these values will be known to
have been created as booleans.  C<is_bool> will tell you whether a value was
known to have been created as a boolean.

=item builtin::weaken, builtin::unweaken, builtin::is_weak

These functions will, respectively: weaken a reference; strengthen a reference;
and return whether a reference is weak.  (A weak reference is not counted for
garbage collection purposes.  See L<perlref>.)  These can take the place of
some similar routines in L<Scalar::Util>.

=item builtin::blessed, builtin::refaddr, builtin::reftype

These functions provide more data about references (or non-references,
actually!) and can take the place of similar routines found in L<Scalar::Util>.

=item builtin::ceil, builtin::floor

C<ceil> returns the smallest integer greater than or equal to its argument.
C<floor> returns the largest integer less than or equal to its argument.  These
can take the place of similar routines found in L<POSIX>.

=back

=head2 C<defer> blocks (experimental)

This release adds support for C<defer> blocks, which are blocks of code
prefixed by the C<defer> modifier. They provide a section of code which runs
at a later time, during scope exit.

In brief, when a C<defer> block is reached at runtime, its body is set aside to
be run when the enclosing scope is exited.  It is unlike a UNITCHECK (among
other reasons) in that if the block I<containing> the C<defer> block is exited
before the block is reached, it will not be run.

C<defer> blocks can be used to take the place of "scope guard" objects where an
object is passed a code block to be run by its destructor.

For more information, see L<perlsyn/"defer blocks">.

=head2 try/catch can now have a C<finally> block (experimental)

The experimental C<try>/C<catch> syntax has been extended to support an
optional third block introduced by the C<finally> keyword.

    try {
        attempt();
        print "Success\n";
    }
    catch ($e) {
        print "Failure\n";
    }
    finally {
        print "This happens regardless\n";
    }

This provides code which runs at the end of the C<try>/C<catch> construct,
even if aborted by an exception or control-flow keyword. They are similar
to C<defer> blocks.

For more information, see L<perlsyn/"Try Catch Exception Handling">.

=head2 non-ASCII delimiters for quote-like operators (experimental)

Perl traditionally has allowed just four pairs of string/pattern
delimiters: S<C<( )>> S<C<{ }>> S<C<[ ]>> and S<C<< < > >>>, all in the
ASCII range.  Unicode has hundreds more possibilities, and using this
feature enables many of them.  When enabled, you can say S<C<qr« »>> for
example, or S<C<use utf8; q𝄃string𝄂>>.  See L<feature/The
'extra_paired_delimiters' feature> for details.

=head2 @_ is now experimental within signatured subs

Even though subroutine signatures are now stable, use of the legacy arguments
array (C<@_>) with a subroutine that has a signature I<remains> experimental,
with its own warning category.  Silencing the C<experimental::signatures>
warning category is not sufficient to dismiss this.  The new warning is emitted
with the category name C<experimental::args_array_with_signatures>.

Any subroutine that has a signature and tries to make use of the defaults
argument array or an element thereof (C<@_> or C<$_[INDEX]>), either
explicitly or implicitly (such as C<shift> or C<pop> with no argument) will
provoke a warning at compile-time:

    use v5.36;

    sub f ($x, $y = 123) {
      say "The first argument is $_[0]";
    }

Z<>

    Use of @_ in array element with signatured subroutine is experimental
    at file.pl line 4.

The behaviour of code which attempts to do this is no longer specified, and
may be subject to change in a future version.

=head1 Incompatible Changes

=head2 A physically empty sort is now a compile-time error

    @a = sort @empty; # unaffected
    @a = sort;        # now a compile-time error
    @a = sort ();     # also a compile-time error

A bare sort used to be a weird way to create an empty list; now it croaks
at compile time. This change is intended to free up some of the syntax space
for possible future enhancements to C<sort>.

=head1 Deprecations

=head2 C<use VERSION> (where VERSION is below v5.11) after C<use v5.11> is deprecated

When in the scope of C<use v5.11> or later, a C<use vX> line where I<X> is
lower than v5.11 will now issue a warning:

    Downgrading a use VERSION declaration to below v5.11 is deprecated

For example:

    use v5.14;
    say "The say statement is permitted";
    use v5.8;                               # This will print a warning
    print "We must use print\n";

This is because the Perl team plans to change the behavior in this case.  Since
Perl v5.12 (and parts of v5.11), strict is enabled I<unless it had previously
been disabled>.  In other words:

    no strict;
    use v5.12;  # will not enable strict, because "no strict" preceded it
    $x = 1;     # permitted, despite no "my" declaration

In the future, this behavior will be eliminated and C<use VERSION> will
I<always> enable strict for versions v5.12 and later.

Code which wishes to mix versions in this manner should use lexical scoping
with block syntax to ensure that the differently versioned regions remain
lexically isolated.

    {
        use v5.14;
        say "The say statement is permitted";
    }

    {
        use v5.8;                           # No warning is emitted
        print "We must use print\n";
    }

Of course, this is probably not something you ever need to do!  If the first
block compiles, it means you're using perl v5.14.0 or later.

=head1 Performance Enhancements

=over 4

=item *

We now probe for compiler support for C11 thread local storage, and where
available use this for "implicit context" for XS extensions making API calls for
a threaded Perl build.  This requires fewer function calls at the C level than
POSIX thread specific storage. We continue to use the the pthreads approach if
the C11 approach is not available.

F<Configure> run with the defaults will build an unthreaded Perl (which is
slightly faster), but most operating systems ship a threaded Perl.

=item *

Perl can now be configured to no longer allocate keys for large hashes
from the shared string table.

The same internal datatype (C<PVHV>) is used for all of

=over 4

=item *

Symbol tables

=item *

Objects (by default)

=item *

Associative arrays

=back

The shared string table was originally added to improve performance for blessed
hashes used as objects, because every object instance has the same keys, so it
is an optimisation to share memory between them. It also makes sense for symbol
tables, where derived classes will have the same keys (typically method names),
and the OP trees built for method calls can also share memory. The shared
string table behaves roughly like a cache for hash keys.

But for hashes actually used as associative arrays - mapping keys to values -
typically the keys are not re-used in other hashes. For example, "seen" hashes
are keyed by object IDs (or addresses), and logically these keys won't repeat
in other hashes.

Storing these "used just once" keys in the shared string table increases CPU
and RAM use for no gain. For such keys the shared string table behaves as a
cache with a 0% hit rate. Storing all the keys there increases the total size
of the shared string table, as well as increasing the number of times it is
resized as it grows. B<Worse> - in any environment that has "copy on write"
memory for child process (such as a pre-forking server), the memory pages used
for the shared string table rapidly need to be copied as the child process
manipulates hashes. Hence if most of the shared string table is such that keys
are used only in one place, there is no benefit from re-use within the perl
interpreter, but a high cost due to more pages for the OS to copy.

The perl interpreter can now be Configured to disable shared hash keys
for "large" hashes (that are neither objects nor symbol tables).  To do
so, add C<-Accflags='-DPERL_USE_UNSHARED_KEYS_IN_LARGE_HASHES'> to
your F<Configure> options.  "Large" is a heuristic -- currently the
heuristic is that sharing is disabled when adding a key to a hash
triggers allocation of more storage, and the hash has more than 42 keys.

This B<might> cause slightly increased memory usage for programs that create
(unblessed) data structures that contain multiple large hashes that share the
same keys. But generally our testing suggests that for the specific cases
described it is a win, and other code is unaffected.

=item *

In certain scenarios, creation of new scalars is now noticeably faster.

For example, the following code is now executing ~30% faster:

    $str = "A" x 64;
    for (0..1_000_000) {
        @svs = split //, $str
    }

(You can read more about this one in L<[perl
#19414]|https://github.com/Perl/perl5/pull/19414>.)

=back

=head1 Modules and Pragmata

=head2 Updated Modules and Pragmata

=over 4

=item *

L<Archive::Tar> has been upgraded from version 2.38 to 2.40.

=item *

L<Attribute::Handlers> has been upgraded from version 1.01 to 1.02.

=item *

L<attributes> has been upgraded from version 0.33 to 0.34.

=item *

L<B> has been upgraded from version 1.82 to 1.83.

=item *

L<B::Concise> has been upgraded from version 1.004 to 1.006.

=item *

L<B::Deparse> has been upgraded from version 1.56 to 1.64.

=item *

L<bignum> has been upgraded from version 0.51 to 0.65.

=item *

L<charnames> has been upgraded from version 1.48 to 1.50.

=item *

L<Compress::Raw::Bzip2> has been upgraded from version 2.101 to 2.103.

=item *

L<Compress::Raw::Zlib> has been upgraded from version 2.101 to 2.105.

=item *

L<CPAN> has been upgraded from version 2.28 to 2.33.

=item *

L<Data::Dumper> has been upgraded from version 2.179 to 2.184.

=item *

L<DB_File> has been upgraded from version 1.855 to 1.857.

=item *

L<Devel::Peek> has been upgraded from version 1.30 to 1.32.

=item *

L<Devel::PPPort> has been upgraded from version 3.62 to 3.68.

=item *

L<diagnostics> has been upgraded from version 1.37 to 1.39.

=item *

L<Digest> has been upgraded from version 1.19 to 1.20.

=item *

L<DynaLoader> has been upgraded from version 1.50 to 1.52.

=item *

L<Encode> has been upgraded from version 3.08 to 3.17.

=item *

L<Errno> has been upgraded from version 1.33 to 1.36.

=item *

L<experimental> has been upgraded from version 0.024 to 0.028.

=item *

L<Exporter> has been upgraded from version 5.76 to 5.77.

=item *

L<ExtUtils::MakeMaker> has been upgraded from version 7.62 to 7.64.

=item *

L<ExtUtils::Miniperl> has been upgraded from version 1.10 to 1.11.

=item *

L<ExtUtils::ParseXS> has been upgraded from version 3.43 to 3.45.

=item *

L<ExtUtils::Typemaps> has been upgraded from version 3.43 to 3.45.

=item *

L<Fcntl> has been upgraded from version 1.14 to 1.15.

=item *

L<feature> has been upgraded from version 1.64 to 1.72.

=item *

L<File::Compare> has been upgraded from version 1.1006 to 1.1007.

=item *

L<File::Copy> has been upgraded from version 2.35 to 2.39.

=item *

L<File::Fetch> has been upgraded from version 1.00 to 1.04.

=item *

L<File::Find> has been upgraded from version 1.39 to 1.40.

=item *

L<File::Glob> has been upgraded from version 1.33 to 1.37.

=item *

L<File::Spec> has been upgraded from version 3.80 to 3.84.

=item *

L<File::stat> has been upgraded from version 1.09 to 1.12.

=item *

L<FindBin> has been upgraded from version 1.52 to 1.53.

=item *

L<GDBM_File> has been upgraded from version 1.19 to 1.23.

=item *

L<Hash::Util> has been upgraded from version 0.25 to 0.28.

=item *

L<Hash::Util::FieldHash> has been upgraded from version 1.21 to 1.26.

=item *

L<HTTP::Tiny> has been upgraded from version 0.076 to 0.080.

=item *

L<I18N::Langinfo> has been upgraded from version 0.19 to 0.21.

=item *

L<if> has been upgraded from version 0.0609 to 0.0610.

=item *

L<IO> has been upgraded from version 1.46 to 1.50.

=item *

IO-Compress has been upgraded from version 2.102 to 2.106.

=item *

L<IPC::Open3> has been upgraded from version 1.21 to 1.22.

=item *

L<JSON::PP> has been upgraded from version 4.06 to 4.07.

=item *

libnet has been upgraded from version 3.13 to 3.14.

=item *

L<Locale::Maketext> has been upgraded from version 1.29 to 1.31.

=item *

L<Math::BigInt> has been upgraded from version 1.999818 to 1.999830.

=item *

L<Math::BigInt::FastCalc> has been upgraded from version 0.5009 to 0.5012.

=item *

L<Math::BigRat> has been upgraded from version 0.2614 to 0.2621.

=item *

L<Module::CoreList> has been upgraded from version 5.20210520 to 5.20220520.

=item *

L<mro> has been upgraded from version 1.25_001 to 1.26.

=item *

L<NEXT> has been upgraded from version 0.68 to 0.69.

=item *

L<Opcode> has been upgraded from version 1.50 to 1.57.

=item *

L<open> has been upgraded from version 1.12 to 1.13.

=item *

L<overload> has been upgraded from version 1.33 to 1.35.

=item *

L<perlfaq> has been upgraded from version 5.20210411 to 5.20210520.

=item *

L<PerlIO> has been upgraded from version 1.11 to 1.12.

=item *

L<Pod::Functions> has been upgraded from version 1.13 to 1.14.

=item *

L<Pod::Html> has been upgraded from version 1.27 to 1.33.

=item *

L<Pod::Simple> has been upgraded from version 3.42 to 3.43.

=item *

L<POSIX> has been upgraded from version 1.97 to 2.03.

=item *

L<re> has been upgraded from version 0.41 to 0.43.

=item *

L<Scalar::Util> has been upgraded from version 1.55 to 1.62.

=item *

L<sigtrap> has been upgraded from version 1.09 to 1.10.

=item *

L<Socket> has been upgraded from version 2.031 to 2.033.

=item *

L<sort> has been upgraded from version 2.04 to 2.05.

=item *

L<Storable> has been upgraded from version 3.23 to 3.26.

=item *

L<Sys::Hostname> has been upgraded from version 1.23 to 1.24.

=item *

L<Test::Harness> has been upgraded from version 3.43 to 3.44.

=item *

L<Test::Simple> has been upgraded from version 1.302183 to 1.302190.

=item *

L<Text::ParseWords> has been upgraded from version 3.30 to 3.31.

=item *

L<Text::Tabs> has been upgraded from version 2013.0523 to 2021.0814.

=item *

L<Text::Wrap> has been upgraded from version 2013.0523 to 2021.0814.

=item *

L<threads> has been upgraded from version 2.26 to 2.27.

=item *

L<threads::shared> has been upgraded from version 1.62 to 1.64.

=item *

L<Tie::Handle> has been upgraded from version 4.2 to 4.3.

=item *

L<Tie::Hash> has been upgraded from version 1.05 to 1.06.

=item *

L<Tie::Scalar> has been upgraded from version 1.05 to 1.06.

=item *

L<Tie::SubstrHash> has been upgraded from version 1.00 to 1.01.

=item *

L<Time::HiRes> has been upgraded from version 1.9767 to 1.9770.

=item *

L<Unicode::Collate> has been upgraded from version 1.29 to 1.31.

=item *

L<Unicode::Normalize> has been upgraded from version 1.28 to 1.31.

=item *

L<Unicode::UCD> has been upgraded from version 0.75 to 0.78.

=item *

L<UNIVERSAL> has been upgraded from version 1.13 to 1.14.

=item *

L<version> has been upgraded from version 0.9928 to 0.9929.

=item *

L<VMS::Filespec> has been upgraded from version 1.12 to 1.13.

=item *

L<VMS::Stdio> has been upgraded from version 2.45 to 2.46.

=item *

L<warnings> has been upgraded from version 1.51 to 1.58.

=item *

L<Win32> has been upgraded from version 0.57 to 0.59.

=item *

L<XS::APItest> has been upgraded from version 1.16 to 1.22.

=item *

L<XS::Typemap> has been upgraded from version 0.18 to 0.19.

=item *

L<XSLoader> has been upgraded from version 0.30 to 0.31.

=back

=head1 Documentation

=head2 New Documentation

=head3 F<Porting/vote_admin_guide.pod>

This document provides the process for administering an election or vote
within the Perl Core Team.

=head2 Changes to Existing Documentation

We have attempted to update the documentation to reflect the changes
listed in this document.  If you find any we have missed, open an issue
at L<https://github.com/Perl/perl5/issues>.

Additionally, the following selected changes have been made:

=head3 L<perlapi>

=over 4

=item *

This has been cleaned up some, and more than 80% of the (previously
many) undocumented functions have now either been documented or deemed
to have been inappropriately marked as API.

As always, Patches Welcome!

=back

=head3 L<perldeprecation>

=over 4

=item *

notes the new location for functions moved from L<Pod::Html> to
L<Pod::Html::Util> that are no longer intended to be used outside of core.

=back

=head3 L<perlexperiment>

=over 4

=item *

notes the C<:win32> IO pseudolayer is removed (this happened in 5.35.2).

=back

=head3 L<perlgov>

=over 4

=item *

The election process has been finetuned to allow the vote to be skipped if there
are no more candidates than open seats.

=item *

A special election is now allowed to be postponed for up to twelve weeks, for
example until a normal election.

=back

=head3 L<perlop>

=over 4

=item *

now notes that an invocant only needs to be an object or class name
for method calls, not for subroutine references.

=back

=head3 L<perlre>

=over 4

=item *

Updated to discourage the use of the /d regexp modifier.

=back

=head3 L<perlrun>

=over 4

=item *

B<-?> is now a synonym for B<-h>

=item *

B<-g> is now a synonym for B<-0777>

=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<Can't "%s" out of a "defer" block|perldiag/"Can't "%s" out of a "defer" block">

(F) An attempt was made to jump out of the scope of a defer block by using
a control-flow statement such as C<return>, C<goto> or a loop control. This is
not permitted.

=item *

L<Can't modify %s in %s|perldiag/"Can't modify %s in %s"> (for scalar
assignment to C<undef>)

Attempting to perform a scalar assignment to C<undef>, for example via
C<undef = $foo;>, previously triggered a fatal runtime error with the
message "L<Modification of a read-only value attempted|perldiag/"Modification of a read-only value attempted">."
It is more helpful to detect such attempted assignments prior to runtime, so
they are now compile time errors, resulting in the message "Can't modify undef
operator in scalar assignment".

=item *

L<panic: newFORLOOP, %s|perldiag/"panic: newFORLOOP, %s">

The parser failed an internal consistency check while trying to parse
a C<foreach> loop.

=back

=head3 New Warnings

=over 4

=item *

L<Built-in function '%s' is experimental|perldiag/"Built-in function '%s' is experimental">

A call is being made to a function in the C<builtin::> namespace, which is
currently experimental.

=item *

L<defer is experimental|perldiag/"defer is experimental">

The C<defer> block modifier is experimental. If you want to use the feature,
disable the warning with C<no warnings 'experimental::defer'>, but know that in
doing so you are taking the risk that your code may break in a future Perl
version.

=item *

L<Downgrading a use VERSION declaration to below v5.11 is deprecated|perldiag/"Downgrading a use VERSION declaration to below v5.11 is deprecated">

This warning is emitted on a C<use VERSION> statement that
requests a version below v5.11 (when the effects of C<use strict> would be
disabled), after a previous declaration of one having a larger number (which
would have enabled these effects)

=item *

L<for my (...) is experimental|perldiag/"for my (...) is experimental">

This warning is emitted if you use C<for> to iterate multiple values at
a time. This syntax is currently experimental and its behaviour may
change in future releases of Perl.

=item *

L<Implicit use of @_ in %s with signatured subroutine is experimental|perldiag/"Implicit use of @_ in %s with signatured subroutine is experimental">

An expression that implicitly involves the C<@_> arguments array was found in
a subroutine that uses a signature.

=item *

L<Use of @_ in %s with signatured subroutine is experimental|perldiag/"Use of @_ in %s with signatured subroutine is experimental">

An expression involving the C<@_> arguments array was found in a subroutine that uses a signature.

=item *

L<Wide character in $0|perldiag/"Wide character in %s">

Attempts to put wide characters into the program name (C<$0>) now provoke this
warning.

=back

=head2 Changes to Existing Diagnostics

=over 4

=item *

L<'E<sol>' does not take a repeat count in %s|perldiag/"'/' does not take a repeat count in %s">

This warning used to not include the C<in %s>.

=item *

L<Subroutine %s redefined|perldiag/"Subroutine %s redefined">

Localized subroutine redefinitions no longer trigger this warning.

=item *

L<unexpected constant lvalue entersub entry via typeE<sol>targ %d:%d"|perldiag/"panic: unexpected constant lvalue entersub entry via type/targ %d:%d"> now has a panic prefix

This makes it consistent with other checks of internal consistency when
compiling a subroutine.

=item *

L<Useless use of sort in scalar context|perldiag/"Useless use of %s in scalar
context"> is now in the new C<scalar> category.

When C<sort> is used in scalar context, it provokes a warning that doing this
is not useful. This warning used to be in the C<void> category. A new category
for warnings about scalar context has now been added, called C<scalar>.

=item *

Removed a number of diagnostics

Many diagnostics that have been removed from the perl core across many years
have now I<also> been removed from the documentation.

=back

=head1 Configuration and Compilation

=over 4

=item *

The Perl C source code now uses some C99 features, which we have verified are
supported by all compilers we target. This means that Perl's headers now
contain some code that is legal in C99 but not C89.

This may cause problems for some XS modules that unconditionally add
C<-Werror=declaration-after-statement> to their C compiler flags if compiling
with gcc or clang. Earlier versions of Perl support long obsolete compilers
that are strict in rejecting certain C99 features, particularly mixed
declarations and code, and hence it makes sense for XS module authors to audit
that their code does not violate this. However, doing this is now only
possible on these earlier versions of Perl, hence these modules need to be
changed to only add this flag for C<<$] < 5.035005>>.

=item *

The makedepend step is now run in parallel by using make

When using MAKEFLAGS=-j8, this significantly reduces the time required for:

    sh ./makedepend MAKE=make cflags

=item *

F<Configure> now tests whether C<< #include <xlocale.h> >> is required
to use the POSIX 1003 thread-safe locale functions or some related
extensions.  This prevents problems where a non-public F<xlocale.h> is
removed in a library update, or F<xlocale.h> isn't intended for public
use. (github L<#18936|https://github.com/Perl/perl5/pull/18936>)

=back

=head1 Testing

Tests were added and changed to reflect the other additions and changes
in this release.

=head1 Platform Support

=head2 Windows

=over 4

=item *

Support for old MSVC++ (pre-VC12) has been removed

These did not support C99 and hence can no longer be used to compile perl.

=item *

Support for compiling perl on Windows using Microsoft Visual Studio 2022
(containing Visual C++ 14.3) has been added.

=item *

The :win32 IO layer has been removed. This experimental replacement for the
:unix layer never reached maturity in its nearly two decades of existence.

=back

=head2 VMS

=over 4

=item C<keys %ENV> on VMS returns consistent results

On VMS entries in the C<%ENV> hash are loaded from the OS environment on
first access, hence the first iteration of C<%ENV> requires the entire
environment to be scanned to find all possible keys. This initialisation had
always been done correctly for full iteration, but previously was not
happening for C<%ENV> in scalar context, meaning that C<scalar %ENV> would
return 0 if called before any other C<%ENV> access, or would only return the
count of keys accessed if there had been no iteration.

These bugs are now fixed - C<%ENV> and C<keys %ENV> in scalar context now
return the correct result - the count of all keys in the environment.

=back

=head2 Discontinued Platforms

=over 4

=item AT&T UWIN

UWIN is a UNIX compatibility layer for Windows.  It was last released
in 2012 and has been superseded by Cygwin these days.

=item DOS/DJGPP

DJGPP is a port of the GNU toolchain to 32-bit x86 systems running
DOS.  The last known attempt to build Perl on it was on 5.20, which
only got as far as building miniperl.

=item NetWare

Support code for Novell NetWare has been removed.  NetWare was a
server operating system by Novell.  The port was last updated in July
2002, and the platform itself in May 2009.

Unrelated changes accidentally broke the build for the NetWare port in
September 2009, and in 12 years no-one has reported this.

=back

=head2 Platform-Specific Notes

=over 4

=item z/OS

This update enables us to build EBCDIC static/dynamic and 31-bit/64-bit
addressing mode Perl. The number of tests that pass is consistent with the
baseline before these updates.

These changes also provide the base support to be able to provide ASCII
static/dynamic and 31-bit/64-bit addressing mode Perl.

The z/OS (previously called OS/390) README was updated to describe ASCII and
EBCDIC builds.

=back

=head1 Internal Changes

=over 4

=item *

Since the removal of PERL_OBJECT in Perl 5.8, PERL_IMPLICIT_CONTEXT and
MULTIPLICITY have been synonymous and they were being used interchangeably.
To simplify the code, all instances of PERL_IMPLICIT_CONTEXT have been
replaced with MULTIPLICITY.

PERL_IMPLICIT_CONTEXT will remain defined for compatibility with XS modules.

=item *

The API constant formerly named C<G_ARRAY>, indicating list context, has now
been renamed to a more accurate C<G_LIST>.  A compatibilty macro C<G_ARRAY> has
been added to allow existing code to work unaffected.  New code should be
written using the new constant instead.  This is supported by C<Devel::PPPort>
version 3.63.

=item *

Macros have been added to F<perl.h> to facilitate version comparisons:
C<PERL_GCC_VERSION_GE>, C<PERL_GCC_VERSION_GT>, C<PERL_GCC_VERSION_LE> and
C<PERL_GCC_VERSION_LT>.

Inline functions have been added to F<embed.h> to determine the position of
the least significant 1 bit in a word: C<lsbit_pos32> and C<lsbit_pos64>.

=item *

C<Perl_ptr_table_clear> has been deleted. This has been marked as deprecated
since v5.14.0 (released in 2011), and is not used by any code on CPAN.

=item *

Added new boolean macros and functions. See L</Stable boolean tracking> for
related information and L<perlapi> for documentation.

=over 4

=item *

sv_setbool

=item *

sv_setbool_mg

=item *

SvIsBOOL

=back

=item *

Added 4 missing functions for dealing with RVs:

=over 4

=item *

sv_setrv_noinc

=item *

sv_setrv_noinc_mg

=item *

sv_setrv_inc

=item *

sv_setrv_inc_mg

=back

=item *

C<xs_handshake()>'s two failure modes now provide distinct messages.

=item *

Memory for hash iterator state (C<struct xpvhv_aux>) is now allocated as part
of the hash body, instead of as part of the block of memory allocated for the
main hash array.

=item *

A new phase_name() interface provides access to the name for each interpreter
phase (i.e., PL_phase value).

=item *

The C<pack> behavior of C<U> has changed for EBCDIC.

=item *

New equality-test functions C<sv_numeq> and C<sv_streq> have been added, along
with C<..._flags>-suffixed variants.  These expose a simple and consistent API
to perform numerical or string comparison which is aware of operator
overloading.

=item *

Reading the string form of an integer value no longer sets the flag C<SVf_POK>.
The string form is still cached internally, and still re-read directly by the
macros C<SvPV(sv)> I<etc> (inline, without calling a C function). XS code that
already calls the APIs to get values will not be affected by this change. XS
code that accesses flags directly instead of using API calls to express its
intent I<might> break, but such code likely is already buggy if passed some
other values, such as floating point values or objects with string overloading.

This small change permits code (such as JSON serializers) to reliably determine
between

=over 4

=item *

a value that was initially B<written> as an integer, but then B<read> as a string

    my $answer = 42;
    print "The answer is $answer\n";

=item *

that same value that was initially B<written> as a string, but then B<read> as an integer

    my $answer = "42";
    print "That doesn't look right\n"
        unless $answer == 6 * 9;

=back

For the first case (originally written as an integer), we now have:

    use Devel::Peek;
    my $answer = 42;
    Dump ($answer);
    my $void = "$answer";
    print STDERR "\n";
    Dump($answer)


    SV = IV(0x562538925778) at 0x562538925788
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 42

    SV = PVIV(0x5625389263c0) at 0x562538925788
      REFCNT = 1
      FLAGS = (IOK,pIOK,pPOK)
      IV = 42
      PV = 0x562538919b50 "42"\0
      CUR = 2
      LEN = 10

For the second (originally written as a string), we now have:

    use Devel::Peek;
    my $answer = "42";
    Dump ($answer);
    my $void = $answer == 6 * 9;
    print STDERR "\n";
    Dump($answer)'


    SV = PV(0x5586ffe9bfb0) at 0x5586ffec0788
      REFCNT = 1
      FLAGS = (POK,IsCOW,pPOK)
      PV = 0x5586ffee7fd0 "42"\0
      CUR = 2
      LEN = 10
      COW_REFCNT = 1

    SV = PVIV(0x5586ffec13c0) at 0x5586ffec0788
      REFCNT = 1
      FLAGS = (IOK,POK,IsCOW,pIOK,pPOK)
      IV = 42
      PV = 0x5586ffee7fd0 "42"\0
      CUR = 2
      LEN = 10
      COW_REFCNT = 1

(One can't rely on the presence or absence of the flag C<SVf_IsCOW> to
determine the history of operations on a scalar.)

Previously both cases would be indistinguishable, with all 4 flags set:

    SV = PVIV(0x55d4d62edaf0) at 0x55d4d62f0930
      REFCNT = 1
      FLAGS = (IOK,POK,pIOK,pPOK)
      IV = 42
      PV = 0x55d4d62e1740 "42"\0
      CUR = 2
      LEN = 10

(and possibly C<SVf_IsCOW>, but not always)

This now means that if XS code I<really> needs to determine which form a value
was first written as, it should implement logic roughly

    if (flags & SVf_IOK|SVf_NOK) && !(flags & SVf_POK)
        serialize as number
    else if (flags & SVf_POK)
        serialize as string
    else
        the existing guesswork ...

Note that this doesn't cover "dualvars" - scalars that report different
values when asked for their string form or number form (such as C<$!>).
Most serialization formats cannot represent such duplicity.

I<The existing guesswork> remains because as well as dualvars, values might
be C<undef>, references, overloaded references, typeglobs and other things that
Perl itself can represent but do not map one-to-one into external formats, so
need some amount of approximation or encapsulation.

=item *

C<sv_dump> (and L<Devel::Peek>’s C<Dump> function) now escapes high-bit
octets in the PV as hex rather than octal. Since most folks understand hex
more readily than octal, this should make these dumps a bit more legible.
This does B<not> affect any other diagnostic interfaces like C<pv_display>.

=back

=head1 Selected Bug Fixes

=over 4

=item *

utime() now correctly sets errno/C<$!> when called on a closed handle.

=item *

The flags on the OPTVAL parameter to setsockopt() were previously
checked before magic was called, possibly treating a numeric value as
a packed buffer or vice versa.  It also ignored the UTF-8 flag,
potentially treating the internal representation of an upgraded SV as
the bytes to supply to the setsockopt() system call.  (github L<#18660|https://github.com/Perl/perl5/issues/18660>)

=item *

Only set IOKp, not IOK on $) and $(.
This was issue L<#18955|https://github.com/Perl/perl5/issues/18955>: This will prevent serializers from serializing these
variables as numbers (which loses the additional groups).
This restores behaviour from 5.16

=item *

Use of the C<mktables> debugging facility would cause perl to croak since
v5.31.10; this problem has now been fixed.

=item *

C<makedepend> logic is now compatible with BSD make (fixes
L<GH #19046|https://github.com/Perl/perl5/issues/19046>).

=item *

Calling C<untie> on a tied hash that is partway through iteration now frees the
iteration state immediately.

Iterating a tied hash causes perl to store a copy of the current hash key to
track the iteration state, with this stored copy passed as the second parameter
to C<NEXTKEY>. This internal state is freed immediately when tie hash iteration
completes, or if the hash is destroyed, but due to an implementation oversight,
it was not freed if the hash was untied. In that case, the internal copy of the
key would persist until the earliest of

=over 4

=item 1

C<tie> was called again on the same hash

=item 2

The (now untied) hash was iterated (ie passed to any of C<keys>, C<values> or
C<each>)

=item 3

The hash was destroyed.

=back

This inconsistency is now fixed - the internal state is now freed immediately by
C<untie>.

As the precise timing of this behaviour can be observed with pure Perl code
(the timing of C<DESTROY> on objects returned from C<FIRSTKEY> and C<NEXTKEY>)
it's just possible that some code is sensitive to it.

=item *

The C<Internals::getcwd()> function added for bootstrapping miniperl
in perl 5.30.0 is now only available in miniperl. [github #19122]

=item *

Setting a breakpoint on a BEGIN or equivalently a C<use> statement
could cause a memory write to a freed C<dbstate> op.
[L<GH #19198|https://github.com/Perl/perl5/issues/19198>]

=item *

When bareword filehandles are disabled, the parser was interpreting
any bareword as a filehandle, even when immediatey followed by parens.

=back

=head1 Errata From Previous Releases

=over 4

=item *

L<perl5300delta> mistakenly identified a CVE whose correct identification is
CVE-2015-1592.

=back

=head1 Obituaries

Raun "Spider" Boardman (SPIDB on CPAN), author of at least 66 commits to the
Perl 5 core distribution between 1996 and 2002, passed away May 24, 2021 from
complications of COVID.  He will be missed.

David H. Adler (DHA) passed away on November 16, 2021.  In 1997, David
co-founded NY.pm, the first Perl user group, and in 1998 co-founded Perl
Mongers to help establish other user groups across the globe.  He was a
frequent attendee at Perl conferences in both North America and Europe and well
known for his role in organizing I<Bad Movie Night> celebrations at those
conferences.  He also contributed to the work of the Perl Foundation, including
administering the White Camel awards for community service.  He will be missed.

=head1 Acknowledgements

Perl 5.36.0 represents approximately a year of development since Perl
5.34.0 and contains approximately 250,000 lines of changes across 2,000
files from 82 authors.

Excluding auto-generated files, documentation and release tools, there were
approximately 190,000 lines of changes to 1,300 .pm, .t, .c and .h files.

Perl continues to flourish into its fourth decade thanks to a vibrant
community of users and developers. The following people are known to have
contributed the improvements that became Perl 5.36.0:

Alyssa Ross, Andrew Fresh, Aristotle Pagaltzis, Asher Mancinelli, Atsushi
Sugawara, Ben Cornett, Bernd, Biswapriyo Nath, Brad Barden, Bram, Branislav
Zahradník, brian d foy, Chad Granum, Chris 'BinGOs' Williams, Christian
Walde (Mithaldu), Christopher Yeleighton, Craig A. Berry, cuishuang, Curtis
Poe, Dagfinn Ilmari Mannsåker, Dan Book, Daniel Laügt, Dan Jacobson, Dan
Kogai, Dave Cross, Dave Lambley, David Cantrell, David Golden, David
Marshall, David Mitchell, E. Choroba, Eugen Konkov, Felipe Gasper, François
Perrad, Graham Knop, H.Merijn Brand, Hugo van der Sanden, Ilya Sashcheka,
Ivan Panchenko, Jakub Wilk, James E Keenan, James Raspass, Karen Etheridge,
Karl Williamson, Leam Hall, Leon Timmermans, Magnus Woldrich, Matthew
Horsfall, Max Maischein, Michael G Schwern, Michiel Beijen, Mike Fulton,
Neil Bowers, Nicholas Clark, Nicolas R, Niyas Sait, Olaf Alders, Paul Evans,
Paul Marquess, Petar-Kaleychev, Pete Houston, Renee Baecker, Ricardo Signes,
Richard Leach, Robert Rothenberg, Sawyer X, Scott Baker, Sergey Poznyakoff,
Sergey Zhmylove, Sisyphus, Slaven Rezic, Steve Hay, Sven Kirmess, TAKAI
Kousuke, Thibault Duponchelle, Todd Rinaldo, Tomasz Konojacki, Tomoyuki
Sadahiro, Tony Cook, Unicode Consortium, Yves Orton, Михаил
Козачков.

The list above is almost certainly incomplete as it is automatically
generated from version control history. In particular, it does not include
the names of the (very much appreciated) contributors who reported issues to
the Perl bug tracker.

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

For a more complete list of all of Perl's historical contributors, please

=head1 Reporting Bugs

If you find what you think is a bug, you might check the perl bug database
at L<https://github.com/Perl/perl5/issues>.  There may also be information at
L<http://www.perl.org/>, the Perl Home Page.

If you believe you have an unreported bug, please open an issue at
L<https://github.com/Perl/perl5/issues>.  Be sure to trim your bug down to a
tiny but sufficient test case.

If the bug you are reporting has security implications which make it
inappropriate to send to a public issue tracker, then see
L<perlsec/SECURITY VULNERABILITY CONTACT INFORMATION>
for details of how to report the issue.

=head1 Give Thanks

If you wish to thank the Perl 5 Porters for the work we had done in Perl 5,
you can do so by running the C<perlthanks> program:

    perlthanks

This will send an email to the Perl 5 Porters list with your show of thanks.

=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