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perl5101delta - Man Page

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:

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

        # matches in 5.8.x, doesn't match in 5.10.0
        $re = qr/^bar/; "foo\nbar" =~ /$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 <http://www.unicode.org/versions/Unicode5.1.0/#Notable_Changes> 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

    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


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.


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


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


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

Pragmata Changes


Upgraded from version 0.08 to 0.09.


Upgraded from version 1.02 to 1.03.


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


Upgraded from version 0.22 to 0.23.


Upgraded from version 0.22 to 0.23.


Upgraded from version 0.22 to 0.23.


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}".


Upgraded from version 1.13 to 1.17.


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.


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


Upgraded from version 0.5565 to 0.62.


Upgraded from version 1.06 to 1.07.


Upgraded from version 1.06 to 1.07.


See "The overloading pragma" above.


Upgraded from version 0.74 to 0.77.

Updated Modules


Upgraded from version 0.24 to 0.34.


Upgraded from version 1.38 to 1.52.


Upgraded from version 0.79 to 0.85.


Upgraded from version 5.63 to 5.68.


Upgraded from version 1.05 to 1.06.


Upgraded from version 1.17 to 1.22.


Upgraded from version 1.05 to 1.11.


Upgraded from version 0.83 to 0.89.


Upgraded from version 1.09 to 1.11.


Upgraded from version 1.01 to 1.02.


Upgraded from version 1.10 to 1.11.


Upgraded from version 1.08 to 1.11.


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


Upgraded from version 2.008 to 2.020.


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.


Upgraded from version 0.06_02 to 0.36.


Upgraded from version 3.25_01 to 3.30.


Upgraded from version 2.121_14 to 2.124.


Upgraded from version 1.01 to 1.02.


Upgraded from version 1.816_1 to 1.820.


Upgraded from version 3.13 to 3.19.


Upgraded from version 2.36_01 to 2.39.


Upgraded from version 5.45 to 5.47.


Upgraded from version 1.01 to 1.03.


Upgraded from version 1.12 to 1.13.


Upgraded from version 1.08 to 1.10.


Upgraded from version 2.23 to 2.35.


Upgraded from version 1.10 to 1.11.


Upgraded from version 5.62 to 5.63.


Upgraded from version 0.21 to 0.2602.


Upgraded from version 1.13 to 1.16.


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


Upgraded from version 1.27 to 1.28.


Upgraded from version 1.44 to 1.54.


Upgraded from version 6.42 to 6.55_02.

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


Upgraded from version 1.51_01 to 1.56.


Upgraded from version 2.18_02 to 2.2002.


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


Upgraded from version 2.76 to 2.77.


Upgraded from version 1.1005 to 1.1006.


Upgraded from version 2.11 to 2.14.


Upgraded from version 0.14 to 0.20.


Upgraded from version 1.12 to 1.14.


Upgraded from version 2.04 to 2.07_03.


Upgraded from version 3.2501 to 3.30.


Upgraded from version 1.00 to 1.01.


Upgraded from version 0.18 to 0.22.


Upgraded from version 1.07 to 1.08.


Upgraded from version 2.01 to 2.02.


Upgraded from version 0.82 to 0.84.


Upgraded from version 1.07 to 1.08.


Upgraded from version 1.49 to 1.50.


Upgraded from version 1.08 to 1.09.


Upgraded from version 2.37 to 2.38.


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


Upgraded from version 1.00 to 1.01.


Upgraded from version 1.23_01 to 1.25.

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


Upgraded from version 2.008 to 2.020.


Upgraded from version 1.06 to 1.07.


Upgraded from version 1.27 to 1.28.


Upgraded from version 1.30_01 to 1.31.


Upgraded from version 1.07 to 1.09.


Upgraded from version 0.40_1 to 0.46.


Upgraded from version 1.02 to 1.04.


Upgraded from version 1.05 to 2.01.


Upgraded from version 0.5565 to 0.62.


Upgraded from version 1.19 to 1.21.


Upgraded from version 1.12 to 1.13.


Upgraded from version 0.01 to 0.02.


Upgraded from version 1.59 to 1.60.


Upgraded from version 1.88 to 1.89.


Upgraded from version 0.16 to 0.19.


Upgraded from version 0.21 to 0.22.


Upgraded from version 1.37 to 1.56.


Upgraded from version 1.04 to 1.20.


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


Upgraded from version 0.2808_01 to 0.34_02.


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


Upgraded from version 0.12 to 0.16.


Upgraded from version 0.22 to 0.30.


Upgraded from version 0.01 to 0.02.


Upgraded from version 3.6 to 3.9.


Upgraded from version 1.07 to 1.08.


Upgraded from version 2.33 to 2.36.


Upgraded from version 0.60_01 to 0.64.


Upgraded from version 0.32 to 0.34.


Upgraded from version 1.03 to 1.04.


Upgraded from version 0.01 to 0.02.


Upgraded from version 1.04 to 1.06.


Upgraded from version 0.04 to 0.07.


Upgraded from version 2.16 to 2.22.


Upgraded from version 1.35 to 1.37.


Upgraded from version 3.05 to 3.07.


Upgraded from version 3.08 to 3.13.


Upgraded from version 1.13 to 1.17.


Upgraded from 2.12 to 2.18.


Upgraded from version 1.19 to 1.21.


Upgraded from 1.01 to 1.02.


Upgraded from 1.11 to 1.17.


Upgraded from 1.80 to 1.82.


Upgraded from 2.18 to 2.20.


Upgraded from version 2.13 to 2.14. Please see "Deprecations".


Upgraded from version 1.06 to 1.07.


Upgraded from version 0.22 to 0.27.


Upgraded from version 1.12 to 2.00.


Upgraded from version 1.03 to 1.04.


Upgraded from version 0.18 to 0.20.


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.


Upgraded from version 0.72 to 0.92.


Upgraded from version 3.26 to 3.27.


Upgraded from version 2007.1117 to 2009.0305.


Upgraded from version 2006.1117 to 2009.0305.


Upgraded from version 2.00 to 2.11.


Upgraded from version 2.01 to 2.09.


Upgraded from version 1.67 to 1.72.


Upgraded from version 1.14 to 1.29.


Upgraded from version 1.37 to 1.38.


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


Upgraded from version 1.9711 to 1.9719.


Upgraded from version 1.18 to 1.1901.


Upgraded from version 1.12 to 1.15.


Upgraded from version 1.02 to 1.03.


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.


Upgraded from version 0.34 to 0.39.


Upgraded from version 0.1001_01 to 0.1101.


Upgraded from version 0.08 to 0.10.

Utility Changes


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


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.


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


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


This describes the new interface for pluggable Method Resolution Orders.


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.


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


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

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 make_ext.pl. This replaces the Unix-specific ext/util/make_ext, VMS-specific make_ext.com and Win32-specific win32/buildext.pl.

Platform Specific Changes


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.


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


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


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.


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


Perl should now build on MirOS BSD.


Hints now supports versions 5.*.

Stratus VOS

Various changes from Stratus have been merged in.


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


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


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

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

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:


Check that the debugger can retain source lines from eval.


Check that bad layers fail.


Check that PerlIO layers are not leaking.


Check that certain special forms of open work.


General PerlIO tests.


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


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


Tests for dbmopen and dbmclose.


Tests for the interaction of index and threads.


Tests for the interaction of esoteric patterns and threads.


Test that qr doesn't leak.


Tests for the interaction of regex recursion and threads.


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


Tests for Unicode properties in regular expressions.


Tests for the interaction of Unicode properties and threads.


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


Check that POSIX character classes behave consistently.


Check that exportable re functions in universal.c work.


Check that setpgrp works.


Tests for the interaction of substr and threads.


Check that upgrading and assigning scalars works.


Check that Unicode in the lexer works.


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.


The following items are now deprecated.


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 http://rt.perl.org/perlbug/ .  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 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 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.

See Also

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.


2024-04-17 perl v5.38.2 Perl Programmers Reference Guide