perl5101delta - 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

       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

   "~~" 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

       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
   <> 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

   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.

   "B" 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

       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

       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
       "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

       This describes the new interface for pluggable Method Resolution

       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

   *   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

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

       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 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.

   MirOS BSD
       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.

   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

   *   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

   *   "$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.

           my $byte = chr(192);
           my $utf8 = chr(192); utf8::upgrade($utf8);
           $utf8 =~ /$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

   *   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

           ("ab" x 32768) =~ /^(ab)*$/

   *   "shmget" was limited to a 32 bit segment size on a 64 bit OS [RT

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

           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.:

            *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

   *   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

   *   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

   *   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

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

   *   Use of a UTF-8 "tr//" within a closure could cause a segfault [RT

   *   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

   *   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

   *   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]:

           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

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

   *   "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

   *   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

       Two flag bits are currently supported.

           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.

           Call "sv_2mortal()" on the new SV.

       There is also a wrapper that takes constant strings,

   *   The function "Perl_croak_xs_usage" has been added as a wrapper to

   *   The functions "PerlIO_find_layer" and "PerlIO_list_alloc" are now

   *   "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

   *   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

   *   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:

       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

       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.

   *   "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

           foo { ... $_ ...} list

   *   The "charnames" pragma may generate a run-time error when a regex
       is interpolated [RT #56444]:

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

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

           my $a = "\N{THAI CHARACTER SARA I}";
           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


   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

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

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

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 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.


   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.


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