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Perl 5 version 10.1 documentation
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perldelta - what is new for perl v5.10.1


This document describes differences between the 5.10.0 release and the 5.10.1 release.

If you are upgrading from an earlier release such as 5.8.8, first read the perl5100delta, which describes differences between 5.8.8 and 5.10.0

Incompatible Changes

Switch statement changes

The handling of complex expressions by the given /when switch statement has been enhanced. There are two new cases where when now interprets its argument as a boolean, instead of an expression to be used in a smart match:

  • flip-flop operators

    The .. and ... flip-flop operators are now evaluated in boolean context, following their usual semantics; see Range Operators in perlop.

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

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

    1. when (/^=begin/ .. /^=end/) {
    2. # do something
    3. }
  • defined-or operator

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

The next section details more changes brought to the semantics to the smart match operator, that naturally also modify the behaviour of the switch statements where smart matching is implicitly used.

Smart match changes

Changes to type-based dispatch

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

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

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

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

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

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

The full dispatch table for the smart match operator is given in Smart matching in detail in perlsyn.

Smart match and overloading

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

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

Other incompatible changes

  • The semantics of use feature :5.10* have changed slightly. See Modules and Pragmata for more information.

  • It is now a run-time error to use the smart match operator ~~ with an object that has no overload defined for it. (This way ~~ will not break encapsulation by matching against the object's internal representation as a reference.)

  • The version control system used for the development of the perl interpreter has been switched from Perforce to git. This is mainly an internal issue that only affects people actively working on the perl core; but it may have minor external visibility, for example in some of details of the output of perl -V . See perlrepository for more information.

  • The internal structure of the ext/ directory in the perl source has been reorganised. In general, a module Foo::Bar whose source was stored under ext/Foo/Bar/ is now located under ext/Foo-Bar/. Also, some modules have been moved from lib/ to ext/. This is purely a source tarball change, and should make no difference to the compilation or installation of perl, unless you have a very customised build process that explicitly relies on this structure, or which hard-codes the nonxs_ext Configure parameter. Specifically, this change does not by default alter the location of any files in the final installation.

  • As part of the Test::Harness 2.x to 3.x upgrade, the experimental Test::Harness::Straps module has been removed. See Updated Modules for more details.

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

  • Module::CoreList no longer contains the %:patchlevel hash.

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

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

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

Core Enhancements

Unicode Character Database 5.1.0

The copy of the Unicode Character Database included in Perl 5.10.1 has been updated to 5.1.0 from 5.0.0. See for the notable changes.

A proper interface for pluggable Method Resolution Orders

As of Perl 5.10.1 there is a new interface for plugging and using method resolution orders other than the default (linear depth first search). The C3 method resolution order added in 5.10.0 has been re-implemented as a plugin, without changing its Perl-space interface. See perlmroapi for more information.

The overloading pragma

This pragma allows you to lexically disable or enable overloading for some or all operations. (Yuval Kogman)

Parallel tests

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

  1. TEST_JOBS=3 make test_harness # Run 3 tests in parallel

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

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

DTrace support

Some support for DTrace has been added. See "DTrace support" in INSTALL.

Support for configure_requires in CPAN module metadata

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

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

Modules and Pragmata

New Modules and Pragmata

  • autodie

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

  • Compress::Raw::Bzip2

    This has been added to the core (version 2.020).

  • parent

    This pragma establishes an ISA relationship with base classes at compile time. It provides the key feature of base without the feature creep.

  • Parse::CPAN::Meta

    This has been added to the core (version 1.39).

Pragmata Changes

  • attributes

    Upgraded from version 0.08 to 0.09.

  • attrs

    Upgraded from version 1.02 to 1.03.

  • base

    Upgraded from version 2.13 to 2.14. See parent for a replacement.

  • bigint

    Upgraded from version 0.22 to 0.23.

  • bignum

    Upgraded from version 0.22 to 0.23.

  • bigrat

    Upgraded from version 0.22 to 0.23.

  • charnames

    Upgraded from version 1.06 to 1.07.

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

  • constant

    Upgraded from version 1.13 to 1.17.

  • feature

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

  • fields

    Upgraded from version 2.13 to 2.14 (this was just a version bump; there were no functional changes).

  • lib

    Upgraded from version 0.5565 to 0.62.

  • open

    Upgraded from version 1.06 to 1.07.

  • overload

    Upgraded from version 1.06 to 1.07.

  • overloading

    See The overloading pragma above.

  • version

    Upgraded from version 0.74 to 0.77.

Updated Modules

  • Archive::Extract

    Upgraded from version 0.24 to 0.34.

  • Archive::Tar

    Upgraded from version 1.38 to 1.52.

  • Attribute::Handlers

    Upgraded from version 0.79 to 0.85.

  • AutoLoader

    Upgraded from version 5.63 to 5.68.

  • AutoSplit

    Upgraded from version 1.05 to 1.06.

  • B

    Upgraded from version 1.17 to 1.22.

  • B::Debug

    Upgraded from version 1.05 to 1.11.

  • B::Deparse

    Upgraded from version 0.83 to 0.89.

  • B::Lint

    Upgraded from version 1.09 to 1.11.

  • B::Xref

    Upgraded from version 1.01 to 1.02.

  • Benchmark

    Upgraded from version 1.10 to 1.11.

  • Carp

    Upgraded from version 1.08 to 1.11.

  • CGI

    Upgraded from version 3.29 to 3.43. (also includes the "default_value for popup_menu()" fix from 3.45).

  • Compress::Zlib

    Upgraded from version 2.008 to 2.020.

  • CPAN

    Upgraded from version 1.9205 to 1.9402. CPAN::FTP has a local fix to stop it being too verbose on download failure.


    Upgraded from version 0.84 to 0.88.

  • CPANPLUS::Dist::Build

    Upgraded from version 0.06_02 to 0.36.

  • Cwd

    Upgraded from version 3.25_01 to 3.30.

  • Data::Dumper

    Upgraded from version 2.121_14 to 2.124.

  • DB

    Upgraded from version 1.01 to 1.02.

  • DB_File

    Upgraded from version 1.816_1 to 1.820.

  • Devel::PPPort

    Upgraded from version 3.13 to 3.19.

  • Digest::MD5

    Upgraded from version 2.36_01 to 2.39.

  • Digest::SHA

    Upgraded from version 5.45 to 5.47.

  • DirHandle

    Upgraded from version 1.01 to 1.03.

  • Dumpvalue

    Upgraded from version 1.12 to 1.13.

  • DynaLoader

    Upgraded from version 1.08 to 1.10.

  • Encode

    Upgraded from version 2.23 to 2.35.

  • Errno

    Upgraded from version 1.10 to 1.11.

  • Exporter

    Upgraded from version 5.62 to 5.63.

  • ExtUtils::CBuilder

    Upgraded from version 0.21 to 0.2602.

  • ExtUtils::Command

    Upgraded from version 1.13 to 1.16.

  • ExtUtils::Constant

    Upgraded from 0.20 to 0.22. (Note that neither of these versions are available on CPAN.)

  • ExtUtils::Embed

    Upgraded from version 1.27 to 1.28.

  • ExtUtils::Install

    Upgraded from version 1.44 to 1.54.

  • ExtUtils::MakeMaker

    Upgraded from version 6.42 to 6.55_02.

    Note that ExtUtils::MakeMaker::bytes and ExtUtils::MakeMaker::vmsish have been removed from this distribution.

  • ExtUtils::Manifest

    Upgraded from version 1.51_01 to 1.56.

  • ExtUtils::ParseXS

    Upgraded from version 2.18_02 to 2.2002.

  • Fatal

    Upgraded from version 1.05 to 2.06_01. See also the new pragma autodie .

  • File::Basename

    Upgraded from version 2.76 to 2.77.

  • File::Compare

    Upgraded from version 1.1005 to 1.1006.

  • File::Copy

    Upgraded from version 2.11 to 2.14.

  • File::Fetch

    Upgraded from version 0.14 to 0.20.

  • File::Find

    Upgraded from version 1.12 to 1.14.

  • File::Path

    Upgraded from version 2.04 to 2.07_03.

  • File::Spec

    Upgraded from version 3.2501 to 3.30.

  • File::stat

    Upgraded from version 1.00 to 1.01.

  • File::Temp

    Upgraded from version 0.18 to 0.22.

  • FileCache

    Upgraded from version 1.07 to 1.08.

  • FileHandle

    Upgraded from version 2.01 to 2.02.

  • Filter::Simple

    Upgraded from version 0.82 to 0.84.

  • Filter::Util::Call

    Upgraded from version 1.07 to 1.08.

  • FindBin

    Upgraded from version 1.49 to 1.50.

  • GDBM_File

    Upgraded from version 1.08 to 1.09.

  • Getopt::Long

    Upgraded from version 2.37 to 2.38.

  • Hash::Util::FieldHash

    Upgraded from version 1.03 to 1.04. This fixes a memory leak.

  • I18N::Collate

    Upgraded from version 1.00 to 1.01.

  • IO

    Upgraded from version 1.23_01 to 1.25.

    This makes non-blocking mode work on Windows in IO::Socket::INET [CPAN #43573].

  • IO::Compress::*

    Upgraded from version 2.008 to 2.020.

  • IO::Dir

    Upgraded from version 1.06 to 1.07.

  • IO::Handle

    Upgraded from version 1.27 to 1.28.

  • IO::Socket

    Upgraded from version 1.30_01 to 1.31.

  • IO::Zlib

    Upgraded from version 1.07 to 1.09.

  • IPC::Cmd

    Upgraded from version 0.40_1 to 0.46.

  • IPC::Open3

    Upgraded from version 1.02 to 1.04.

  • IPC::SysV

    Upgraded from version 1.05 to 2.01.

  • lib

    Upgraded from version 0.5565 to 0.62.

  • List::Util

    Upgraded from version 1.19 to 1.21.

  • Locale::MakeText

    Upgraded from version 1.12 to 1.13.

  • Log::Message

    Upgraded from version 0.01 to 0.02.

  • Math::BigFloat

    Upgraded from version 1.59 to 1.60.

  • Math::BigInt

    Upgraded from version 1.88 to 1.89.

  • Math::BigInt::FastCalc

    Upgraded from version 0.16 to 0.19.

  • Math::BigRat

    Upgraded from version 0.21 to 0.22.

  • Math::Complex

    Upgraded from version 1.37 to 1.56.

  • Math::Trig

    Upgraded from version 1.04 to 1.20.

  • Memoize

    Upgraded from version 1.01_02 to 1.01_03 (just a minor documentation change).

  • Module::Build

    Upgraded from version 0.2808_01 to 0.34_02.

  • Module::CoreList

    Upgraded from version 2.13 to 2.18. This release no longer contains the %Module::CoreList::patchlevel hash.

  • Module::Load

    Upgraded from version 0.12 to 0.16.

  • Module::Load::Conditional

    Upgraded from version 0.22 to 0.30.

  • Module::Loaded

    Upgraded from version 0.01 to 0.02.

  • Module::Pluggable

    Upgraded from version 3.6 to 3.9.

  • NDBM_File

    Upgraded from version 1.07 to 1.08.

  • Net::Ping

    Upgraded from version 2.33 to 2.36.

  • NEXT

    Upgraded from version 0.60_01 to 0.64.

  • Object::Accessor

    Upgraded from version 0.32 to 0.34.

  • OS2::REXX

    Upgraded from version 1.03 to 1.04.

  • Package::Constants

    Upgraded from version 0.01 to 0.02.

  • PerlIO

    Upgraded from version 1.04 to 1.06.

  • PerlIO::via

    Upgraded from version 0.04 to 0.07.

  • Pod::Man

    Upgraded from version 2.16 to 2.22.

  • Pod::Parser

    Upgraded from version 1.35 to 1.37.

  • Pod::Simple

    Upgraded from version 3.05 to 3.07.

  • Pod::Text

    Upgraded from version 3.08 to 3.13.


    Upgraded from version 1.13 to 1.17.

  • Safe

    Upgraded from 2.12 to 2.18.

  • Scalar::Util

    Upgraded from version 1.19 to 1.21.

  • SelectSaver

    Upgraded from 1.01 to 1.02.

  • SelfLoader

    Upgraded from 1.11 to 1.17.

  • Socket

    Upgraded from 1.80 to 1.82.

  • Storable

    Upgraded from 2.18 to 2.20.

  • Switch

    Upgraded from version 2.13 to 2.14. Please see Deprecations.

  • Symbol

    Upgraded from version 1.06 to 1.07.

  • Sys::Syslog

    Upgraded from version 0.22 to 0.27.

  • Term::ANSIColor

    Upgraded from version 1.12 to 2.00.

  • Term::ReadLine

    Upgraded from version 1.03 to 1.04.

  • Term::UI

    Upgraded from version 0.18 to 0.20.

  • Test::Harness

    Upgraded from version 2.64 to 3.17.

    Note that one side-effect of the 2.x to 3.x upgrade is that the experimental Test::Harness::Straps module (and its supporting Assert , Iterator , Point and Results modules) have been removed. If you still need this, then they are available in the (unmaintained) Test-Harness-Straps distribution on CPAN.

  • Test::Simple

    Upgraded from version 0.72 to 0.92.

  • Text::ParseWords

    Upgraded from version 3.26 to 3.27.

  • Text::Tabs

    Upgraded from version 2007.1117 to 2009.0305.

  • Text::Wrap

    Upgraded from version 2006.1117 to 2009.0305.

  • Thread::Queue

    Upgraded from version 2.00 to 2.11.

  • Thread::Semaphore

    Upgraded from version 2.01 to 2.09.

  • threads

    Upgraded from version 1.67 to 1.72.

  • threads::shared

    Upgraded from version 1.14 to 1.29.

  • Tie::RefHash

    Upgraded from version 1.37 to 1.38.

  • Tie::StdHandle

    This has documentation changes, and has been assigned a version for the first time: version 4.2.

  • Time::HiRes

    Upgraded from version 1.9711 to 1.9719.

  • Time::Local

    Upgraded from version 1.18 to 1.1901.

  • Time::Piece

    Upgraded from version 1.12 to 1.15.

  • Unicode::Normalize

    Upgraded from version 1.02 to 1.03.

  • Unicode::UCD

    Upgraded from version 0.25 to 0.27.

    charinfo() now works on Unified CJK code points added to later versions of Unicode.

    casefold() has new fields returned to provide both a simpler interface and previously missing information. The old fields are retained for backwards compatibility. Information about Turkic-specific code points is now returned.

    The documentation has been corrected and expanded.


    Upgraded from version 1.04 to 1.05.

  • Win32

    Upgraded from version 0.34 to 0.39.

  • Win32API::File

    Upgraded from version 0.1001_01 to 0.1101.

  • XSLoader

    Upgraded from version 0.08 to 0.10.

Utility Changes

  • h2ph

    Now looks in include-fixed too, which is a recent addition to gcc's search path.

  • h2xs

    No longer incorrectly treats enum values like macros (Daniel Burr).

    Now handles C++ style constants (// ) properly in enums. (A patch from Rainer Weikusat was used; Daniel Burr also proposed a similar fix).


    LVALUE subroutines now work under the debugger.

    The debugger now correctly handles proxy constant subroutines, and subroutine stubs.

  • perlthanks

    Perl 5.10.1 adds a new utility perlthanks, which is a variant of perlbug, but for sending non-bug-reports to the authors and maintainers of Perl. Getting nothing but bug reports can become a bit demoralising: we'll see if this changes things.

New Documentation

  • perlhaiku

    This contains instructions on how to build perl for the Haiku platform.

  • perlmroapi

    This describes the new interface for pluggable Method Resolution Orders.

  • perlperf

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

  • perlrepository

    This describes how to access the perl source using the git version control system.

  • perlthanks

    This describes the new perlthanks utility.

Changes to Existing Documentation

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

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

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

Performance Enhancements

  • A new internal cache means that isa() will often be faster.

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

  • Empty DESTROY methods are no longer called.

Installation and Configuration Improvements

ext/ reorganisation

The layout of directories in ext has been revised. Specifically, all extensions are now flat, and at the top level, with / in pathnames replaced by - , so that ext/Data/Dumper/ is now ext/Data-Dumper/, etc. The names of the extensions as specified to Configure, and as reported by %Config::Config under the keys dynamic_ext , known_extensions , nonxs_ext and static_ext have not changed, and still use /. Hence this change will not have any affect once perl is installed. However, Attribute::Handlers , Safe and mro have now become extensions in their own right, so if you run Configure with options to specify an exact list of extensions to build, you will need to change it to account for this.

For 5.10.2, it is planned that many dual-life modules will have been moved from lib to ext; again this will have no effect on an installed perl, but will matter if you invoke Configure with a pre-canned list of extensions to build.

Configuration improvements

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

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

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

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

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

Compilation improvements

As part of the flattening of ext, all extensions on all platforms are built by This replaces the Unix-specific ext/util/make_ext, VMS-specific and Win32-specific win32/

Platform Specific Changes

  • AIX

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

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

    Hints changes mean that AIX 4.2 should work again.

  • Cygwin

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

  • FreeBSD

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

  • Irix

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

  • Haiku

    Patches from the Haiku maintainers have been merged in. Perl should now build on Haiku.

  • MirOS BSD

    Perl should now build on MirOS BSD.

  • NetBSD

    Hints now supports versions 5.*.

  • Stratus VOS

    Various changes from Stratus have been merged in.

  • Symbian

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

  • Win32

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

  • VMS

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

    VMS now supports getgrgid.

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

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

Selected Bug Fixes

  • 5.10.0 inadvertently disabled an optimisation, which caused a measurable performance drop in list assignment, such as is often used to assign function parameters from @_ . The optimisation has been re-instated, and the performance regression fixed.

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

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

  • The debugger now works with lvalue subroutines.

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

  • crypt() and string complement could return tainted values for untainted arguments [RT #59998].

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

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

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

  • (XS) In various hash functions, passing a pre-computed hash to when the key is UTF-8 might result in an incorrect lookup.

  • (XS) Including XSUB.h before perl.h gave a compile-time error [RT #57176].

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

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

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

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

    1. my $byte = chr(192);
    2. my $utf8 = chr(192); utf8::upgrade($utf8);
    3. $utf8 =~ /$byte|X}/i; # failed in 5.10.0
  • Within UTF8-encoded Perl source files (i.e. where use utf8 is in effect), double-quoted literal strings could be corrupted where a \xNN , \0NNN or \N{} is followed by a literal character with ordinal value greater than 255 [RT #59908].

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

  • Using setpgrp() with no arguments could corrupt the perl stack.

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

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

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

    1. ("ab" x 32768) =~ /^(ab)*$/
  • shmget was limited to a 32 bit segment size on a 64 bit OS [RT #63924].

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

    1. Exiting given via last at line 123
  • On Windows, '.\foo' and '..\foo' were treated differently than './foo' and '../foo' by do and require [RT #63492].

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

    1. *bar=*foo{FORMAT}; # foo format now bad
  • Attempting to coerce a typeglob to a string or number could cause an assertion failure. The correct error message is now generated, Can't coerce GLOB to $type.

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

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

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

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

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

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

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

  • The line numbers for warnings inside elsif are now correct.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

New or Changed Diagnostics

  • panic: sv_chop %s

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

  • Can't locate package %s for the parents of %s

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

  • v-string in use/require is non-portable

    This warning has been removed.

  • Deep recursion on subroutine "%s"

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

Changed Internals

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

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

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

  • The function Perl_sv_insert_flags has been added to augment Perl_sv_insert .

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

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

    Two flag bits are currently supported.

    • SVf_UTF8

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

    • SVs_TEMP

      Call sv_2mortal() on the new SV.

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

  • The function Perl_croak_xs_usage has been added as a wrapper to Perl_croak .

  • The functions PerlIO_find_layer and PerlIO_list_alloc are now exported.

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

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

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

  • Perl_mg_magical() would sometimes incorrectly turn on SvRMAGICAL() . This has been fixed.

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

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

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

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

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

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

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

New Tests

Many modules updated from CPAN incorporate new tests.

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

Some core-specific tests have been added:

  • t/comp/retainedlines.t

    Check that the debugger can retain source lines from eval.

  • t/io/perlio_fail.t

    Check that bad layers fail.

  • t/io/perlio_leaks.t

    Check that PerlIO layers are not leaking.

  • t/io/perlio_open.t

    Check that certain special forms of open work.

  • t/io/perlio.t

    General PerlIO tests.

  • t/io/pvbm.t

    Check that there is no unexpected interaction between the internal types PVBM and PVGV .

  • t/mro/package_aliases.t

    Check that mro works properly in the presence of aliased packages.

  • t/op/dbm.t

    Tests for dbmopen and dbmclose.

  • t/op/index_thr.t

    Tests for the interaction of index and threads.

  • t/op/pat_thr.t

    Tests for the interaction of esoteric patterns and threads.

  • t/op/qr_gc.t

    Test that qr doesn't leak.

  • t/op/reg_email_thr.t

    Tests for the interaction of regex recursion and threads.

  • t/op/regexp_qr_embed_thr.t

    Tests for the interaction of patterns with embedded qr// and threads.

  • t/op/regexp_unicode_prop.t

    Tests for Unicode properties in regular expressions.

  • t/op/regexp_unicode_prop_thr.t

    Tests for the interaction of Unicode properties and threads.

  • t/op/reg_nc_tie.t

    Test the tied methods of Tie::Hash::NamedCapture .

  • t/op/reg_posixcc.t

    Check that POSIX character classes behave consistently.

  • t/op/re.t

    Check that exportable re functions in universal.c work.

  • t/op/setpgrpstack.t

    Check that setpgrp works.

  • t/op/substr_thr.t

    Tests for the interaction of substr and threads.

  • t/op/upgrade.t

    Check that upgrading and assigning scalars works.

  • t/uni/lex_utf8.t

    Check that Unicode in the lexer works.

  • t/uni/tie.t

    Check that Unicode and tie work.

Known Problems

This is a list of some significant unfixed bugs, which are regressions from either 5.10.0 or 5.8.x.

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

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

    1. foo { ... $_ ...} list
  • The charnames pragma may generate a run-time error when a regex is interpolated [RT #56444]:

    1. use charnames ':full';
    2. my $r1 = qr/\N{THAI CHARACTER SARA I}/;
    3. "foo" =~ $r1; # okay
    4. "foo" =~ /$r1+/; # runtime error

    A workaround is to generate the character outside of the regex:

    1. my $a = "\N{THAI CHARACTER SARA I}";
    2. my $r1 = qr/$a/;
  • Some regexes may run much more slowly when run in a child thread compared with the thread the pattern was compiled into [RT #55600].


The following items are now deprecated.

  • Switch is buggy and should be avoided. From perl 5.11.0 onwards, it is intended that any use of the core version of this module will emit a warning, and that the module will eventually be removed from the core (probably in perl 5.14.0). See Switch statements in perlsyn for its replacement.

  • suidperl will be removed in 5.12.0. This provides a mechanism to emulate setuid permission bits on systems that don't support it properly.


Some of the work in this release was funded by a TPF grant.

Nicholas Clark officially retired from maintenance pumpking duty at the end of 2008; however in reality he has put much effort in since then to help get 5.10.1 into a fit state to be released, including writing a considerable chunk of this perldelta.

Steffen Mueller and David Golden in particular helped getting CPAN modules polished and synchronised with their in-core equivalents.

Craig Berry was tireless in getting maint to run under VMS, no matter how many times we broke it for him.

The other core committers contributed most of the changes, and applied most of the patches sent in by the hundreds of contributors listed in AUTHORS.

(Sorry to all the people I haven't mentioned by name).

Finally, thanks to Larry Wall, without whom none of this would be necessary.

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 . There may also be information at , the Perl Home Page.

If you believe you have an unreported bug, please run the 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 perl -V , will be sent off to 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 This points to a closed subscription unarchived mailing list, which includes all the core committers, who be able to help assess the impact of issues, figure out a resolution, and help co-ordinate the release of patches to mitigate or fix the problem across all platforms on which Perl is supported. Please only use this address for security issues in the Perl core, not for modules independently distributed on CPAN.


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

The INSTALL file for how to build Perl.

The README file for general stuff.

The Artistic and Copying files for copyright information.