This section contains a summary of the changes between the 5.6.0 release and the 5.6.1 release. More details about the changes mentioned here may be found in the Changes files that accompany the Perl source distribution. See the perlhack manpage for pointers to online resources where you can inspect the individual patches described by these changes.

Security Issues

suidperl will not run /bin/mail anymore, because some platforms have a /bin/mail that is vulnerable to buffer overflow attacks.

Note that suidperl is neither built nor installed by default in any recent version of perl. Use of suidperl is highly discouraged. If you think you need it, try alternatives such as sudo first. See http://www.courtesan.com/sudo/.

Core bug fixes

This is not an exhaustive list. It is intended to cover only the significant user-visible changes.

UNIVERSAL::isa()

A bug in the caching mechanism used by UNIVERSAL::isa() that affected base.pm has been fixed. The bug has existed since the 5.005 releases, but wasn't tickled by base.pm in those releases.

Memory leaks

Various cases of memory leaks and attempts to access uninitialized memory have been cured. See Known Problems below for further issues.

Numeric conversions

Numeric conversions did not recognize changes in the string value properly in certain circumstances.

In other situations, large unsigned numbers (those above 2**31) could sometimes lose their unsignedness, causing bogus results in arithmetic operations.

Integer modulus on large unsigned integers sometimes returned incorrect values.

Perl 5.6.0 generated ``not a number'' warnings on certain conversions where previous versions didn't.

These problems have all been rectified.

Infinity is now recognized as a number.

qw(a\\b)

In Perl 5.6.0, qw(a\\b) produced a string with two backslashes instead of one, in a departure from the behavior in previous versions. The older behavior has been reinstated.

caller()

caller() could cause core dumps in certain situations. Carp was sometimes affected by this problem.

Bugs in regular expressions

Pattern matches on overloaded values are now handled correctly.

Perl 5.6.0 parsed m/\x{ab}/ incorrectly, leading to spurious warnings. This has been corrected.

The RE engine found in Perl 5.6.0 accidentally pessimised certain kinds of simple pattern matches. These are now handled better.

Regular expression debug output (whether through use re 'debug' or via -Dr) now looks better.

Multi-line matches like "a\nxb\n" =~ /(?!\A)x/m were flawed. The bug has been fixed.

Use of $& could trigger a core dump under some situations. This is now avoided.

Match variables $1 et al., weren't being unset when a pattern match was backtracking, and the anomaly showed up inside /...(?{ ... }).../ etc. These variables are now tracked correctly.

pos() did not return the correct value within s///ge in earlier versions. This is now handled correctly.

``slurp'' mode

readline() on files opened in ``slurp'' mode could return an extra ``'' at the end in certain situations. This has been corrected.

Autovivification of symbolic references to special variables

Autovivification of symbolic references of special variables described in the perlvar manpage (as in ${$num}) was accidentally disabled. This works again now.

Lexical warnings

Lexical warnings now propagate correctly into eval "...".

use warnings qw(FATAL all) did not work as intended. This has been corrected.

Lexical warnings could leak into other scopes in some situations. This is now fixed.

warnings::enabled() now reports the state of $^W correctly if the caller isn't using lexical warnings.

Spurious warnings and errors

Perl 5.6.0 could emit spurious warnings about redefinition of dl_error() when statically building extensions into perl. This has been corrected.

``our'' variables could result in bogus ``Variable will not stay shared'' warnings. This is now fixed.

``our'' variables of the same name declared in two sibling blocks resulted in bogus warnings about ``redeclaration'' of the variables. The problem has been corrected.

glob()

Compatibility of the builtin glob() with old csh-based glob has been improved with the addition of GLOB_ALPHASORT option. See File::Glob.

File::Glob::glob() has been renamed to File::Glob::bsd_glob() because the name clashes with the builtin glob(). The older name is still available for compatibility, but is deprecated.

Spurious syntax errors generated in certain situations, when glob() caused File::Glob to be loaded for the first time, have been fixed.

Tainting

Some cases of inconsistent taint propagation (such as within hash values) have been fixed.

The tainting behavior of sprintf() has been rationalized. It does not taint the result of floating point formats anymore, making the behavior consistent with that of string interpolation.

sort()

Arguments to sort() weren't being provided the right wantarray() context. The comparison block is now run in scalar context, and the arguments to be sorted are always provided list context.

sort() is also fully reentrant, in the sense that the sort function can itself call sort(). This did not work reliably in previous releases.

#line directives

#line directives now work correctly when they appear at the very beginning of eval "...".

Subroutine prototypes

The (\&) prototype now works properly.

map()

map() could get pathologically slow when the result list it generates is larger than the source list. The performance has been improved for common scenarios.

Debugger

Debugger exit code now reflects the script exit code.

Condition "0" in breakpoints is now treated correctly.

The d command now checks the line number.

$. is no longer corrupted by the debugger.

All debugger output now correctly goes to the socket if RemotePort is set.

PERL5OPT

PERL5OPT can be set to more than one switch group. Previously, it used to be limited to one group of options only.

chop()

chop(@list) in list context returned the characters chopped in reverse order. This has been reversed to be in the right order.

Unicode support

Unicode support has seen a large number of incremental improvements, but continues to be highly experimental. It is not expected to be fully supported in the 5.6.x maintenance releases.

substr(), join(), repeat(), reverse(), quotemeta() and string concatenation were all handling Unicode strings incorrectly in Perl 5.6.0. This has been corrected.

Support for tr///CU and tr///UC etc., have been removed since we realized the interface is broken. For similar functionality, see pack in the perlfunc manpage.

The Unicode Character Database has been updated to version 3.0.1 with additions made available to the public as of August 30, 2000.

The Unicode character classes \p{Blank} and \p{SpacePerl} have been added. ``Blank'' is like C isblank(), that is, it contains only ``horizontal whitespace'' (the space character is, the newline isn't), and the ``SpacePerl'' is the Unicode equivalent of \s (\p{Space} isn't, since that includes the vertical tabulator character, whereas \s doesn't.)

If you are experimenting with Unicode support in perl, the development versions of Perl may have more to offer. In particular, I/O layers are now available in the development track, but not in the maintenance track, primarily to do backward compatibility issues. Unicode support is also evolving rapidly on a daily basis in the development track--the maintenance track only reflects the most conservative of these changes.

-bit support
Support for 64-bit platforms has been improved, but continues to be experimental. The level of support varies greatly among platforms.

Compiler

The B Compiler and its various backends have had many incremental improvements, but they continue to remain highly experimental. Use in production environments is discouraged.

The perlcc tool has been rewritten so that the user interface is much more like that of a C compiler.

The perlbc tools has been removed. Use perlcc -B instead.

Lvalue subroutines

There have been various bugfixes to support lvalue subroutines better. However, the feature still remains experimental.

IO::Socket

IO::Socket::INET failed to open the specified port if the service name was not known. It now correctly uses the supplied port number as is.

File::Find

File::Find now chdir()s correctly when chasing symbolic links.

xsubpp

xsubpp now tolerates embedded POD sections.

no Module;

no Module; does not produce an error even if Module does not have an unimport() method. This parallels the behavior of use vis-a-vis import.

Tests

A large number of tests have been added.

Core features

untie() will now call an UNTIE() hook if it exists. See the perltie manpage for details.

The -DT command line switch outputs copious tokenizing information. See the perlrun manpage.

Arrays are now always interpolated in double-quotish strings. Previously, "foo@bar.com" used to be a fatal error at compile time, if an array @bar was not used or declared. This transitional behavior was intended to help migrate perl4 code, and is deemed to be no longer useful. See Arrays now always interpolate into double-quoted strings.

keys(), each(), pop(), push(), shift(), splice() and unshift() can all be overridden now.

my __PACKAGE__ $obj now does the expected thing.

Configuration issues

On some systems (IRIX and Solaris among them) the system malloc is demonstrably better. While the defaults haven't been changed in order to retain binary compatibility with earlier releases, you may be better off building perl with Configure -Uusemymalloc ... as discussed in the INSTALL file.

Configure has been enhanced in various ways:

Minimizes use of temporary files.
By default, does not link perl with libraries not used by it, such as the various dbm libraries. SunOS 4.x hints preserve behavior on that platform.
Support for pdp11-style memory models has been removed due to obsolescence.
Building outside the source tree is supported on systems that have symbolic links. This is done by running
```
    sh /path/to/source/Configure -Dmksymlinks ...
    make all test install
```
in a directory other than the perl source directory. See INSTALL.
Configure -S can be run non-interactively.

Documentation

README.aix, README.solaris and README.macos have been added. README.posix-bc has been renamed to README.bs2000. These are installed as the perlaix manpage, the perlsolaris manpage, the perlmacos manpage, and the perlbs2000 manpage respectively.

The following pod documents are brand new:

    perlclib    Internal replacements for standard C library functions
    perldebtut  Perl debugging tutorial
    perlebcdic  Considerations for running Perl on EBCDIC platforms
    perlnewmod  Perl modules: preparing a new module for distribution
    perlrequick Perl regular expressions quick start
    perlretut   Perl regular expressions tutorial
    perlutil    utilities packaged with the Perl distribution

The INSTALL file has been expanded to cover various issues, such as 64-bit support.

A longer list of contributors has been added to the source distribution. See the file AUTHORS.

Numerous other changes have been made to the included documentation and FAQs.

Bundled modules

The following modules have been added.

B::Concise: Walks Perl syntax tree, printing concise info about ops. See the B::Concise manpage.
File::Temp: Returns name and handle of a temporary file safely. See the File::Temp manpage.
Pod::LaTeX: Converts Pod data to formatted LaTeX. See the Pod::LaTeX manpage.
Pod::Text::Overstrike: Converts POD data to formatted overstrike text. See the Pod::Text::Overstrike manpage.

The following modules have been upgraded.

CGI: CGI v2.752 is now included.
CPAN: CPAN v1.59_54 is now included.
Class::Struct: Various bugfixes have been added.
DB_File: DB_File v1.75 supports newer Berkeley DB versions, among other improvements.
Devel::Peek: Devel::Peek has been enhanced to support dumping of memory statistics, when perl is built with the included malloc().
File::Find: File::Find now supports pre and post-processing of the files in order to sort() them, etc.
Getopt::Long: Getopt::Long v2.25 is included.
IO::Poll: Various bug fixes have been included.
IPC::Open3: IPC::Open3 allows use of numeric file descriptors.
Math::BigFloat: The fmod() function supports modulus operations. Various bug fixes have also been included.
Math::Complex: Math::Complex handles inf, NaN etc., better.
Net::Ping: ping() could fail on odd number of data bytes, and when the echo service isn't running. This has been corrected.
Opcode: A memory leak has been fixed.
Pod::Parser: Version 1.13 of the Pod::Parser suite is included.
Pod::Text: Pod::Text and related modules have been upgraded to the versions in podlators suite v2.08.
SDBM_File: On dosish platforms, some keys went missing because of lack of support for files with ``holes''. A workaround for the problem has been added.
Sys::Syslog: Various bug fixes have been included.
Tie::RefHash: Now supports Tie::RefHash::Nestable to automagically tie hashref values.
Tie::SubstrHash: Various bug fixes have been included.

Platform-specific improvements

The following new ports are now available.

NCR MP-RAS
NonStop-UX

Perl now builds under Amdahl UTS.

Perl has also been verified to build under Amiga OS.

Support for EPOC has been much improved. See README.epoc.

Building perl with -Duseithreads or -Duse5005threads now works under HP-UX 10.20 (previously it only worked under 10.30 or later). You will need a thread library package installed. See README.hpux.

Long doubles should now work under Linux.

MacOS Classic is now supported in the mainstream source package. See README.macos.

Support for MPE/iX has been updated. See README.mpeix.

Support for OS/2 has been improved. See os2/Changes and README.os2.

Dynamic loading on z/OS (formerly OS/390) has been improved. See README.os390.

Support for VMS has seen many incremental improvements, including better support for operators like backticks and system(), and better %ENV handling. See README.vms and the perlvms manpage.

Support for Stratus VOS has been improved. See vos/Changes and README.vos.

Support for Windows has been improved.

fork() emulation has been improved in various ways, but still continues to be experimental. See the perlfork manpage for known bugs and caveats.
%SIG has been enabled under USE_ITHREADS, but its use is completely unsupported under all configurations.
Borland C++ v5.5 is now a supported compiler that can build Perl. However, the generated binaries continue to be incompatible with those generated by the other supported compilers (GCC and Visual C++).
Non-blocking waits for child processes (or pseudo-processes) are supported via waitpid($pid, &POSIX::WNOHANG).
A memory leak in accept() has been fixed.
wait(), waitpid() and backticks now return the correct exit status under Windows 9x.
Trailing new %ENV entries weren't propagated to child processes. This is now fixed.
Current directory entries in %ENV are now correctly propagated to child processes.
Duping socket handles with open(F, ``>&MYSOCK'') now works under Windows 9x.
The makefiles now provide a single switch to bulk-enable all the features enabled in ActiveState ActivePerl (a popular binary distribution).
Win32::GetCwd() correctly returns C:\ instead of C: when at the drive root. Other bugs in chdir() and Cwd::cwd() have also been fixed.
fork() correctly returns undef and sets EAGAIN when it runs out of pseudo-process handles.
ExtUtils::MakeMaker now uses $ENV{LIB} to search for libraries.
UNC path handling is better when perl is built to support fork().
A handle leak in socket handling has been fixed.
send() works from within a pseudo-process.

Unless specifically qualified otherwise, the remainder of this document covers changes between the 5.005 and 5.6.0 releases.

Core Enhancements

Interpreter cloning, threads, and concurrency

Perl 5.6.0 introduces the beginnings of support for running multiple interpreters concurrently in different threads. In conjunction with the perl_clone() API call, which can be used to selectively duplicate the state of any given interpreter, it is possible to compile a piece of code once in an interpreter, clone that interpreter one or more times, and run all the resulting interpreters in distinct threads.

On the Windows platform, this feature is used to emulate fork() at the interpreter level. See the perlfork manpage for details about that.

This feature is still in evolution. It is eventually meant to be used to selectively clone a subroutine and data reachable from that subroutine in a separate interpreter and run the cloned subroutine in a separate thread. Since there is no shared data between the interpreters, little or no locking will be needed (unless parts of the symbol table are explicitly shared). This is obviously intended to be an easy-to-use replacement for the existing threads support.

Support for cloning interpreters and interpreter concurrency can be enabled using the -Dusethreads Configure option (see win32/Makefile for how to enable it on Windows.) The resulting perl executable will be functionally identical to one that was built with -Dmultiplicity, but the perl_clone() API call will only be available in the former.

-Dusethreads enables the cpp macro USE_ITHREADS by default, which in turn enables Perl source code changes that provide a clear separation between the op tree and the data it operates with. The former is immutable, and can therefore be shared between an interpreter and all of its clones, while the latter is considered local to each interpreter, and is therefore copied for each clone.

Note that building Perl with the -Dusemultiplicity Configure option is adequate if you wish to run multiple independent interpreters concurrently in different threads. -Dusethreads only provides the additional functionality of the perl_clone() API call and other support for running cloned interpreters concurrently.

    NOTE: This is an experimental feature.  Implementation details are
    subject to change.

Lexically scoped warning categories

You can now control the granularity of warnings emitted by perl at a finer level using the use warnings pragma. the warnings manpage and the perllexwarn manpage have copious documentation on this feature.

Unicode and UTF-8 support

Perl now uses UTF-8 as its internal representation for character strings. The utf8 and bytes pragmas are used to control this support in the current lexical scope. See the perlunicode manpage, the utf8 manpage and the bytes manpage for more information.

This feature is expected to evolve quickly to support some form of I/O disciplines that can be used to specify the kind of input and output data (bytes or characters). Until that happens, additional modules from CPAN will be needed to complete the toolkit for dealing with Unicode.

    NOTE: This should be considered an experimental feature.  Implementation
    details are subject to change.

Support for interpolating named characters

The new \N escape interpolates named characters within strings. For example, "Hi! \N{WHITE SMILING FACE}" evaluates to a string with a Unicode smiley face at the end.

``our'' declarations

An ``our'' declaration introduces a value that can be best understood as a lexically scoped symbolic alias to a global variable in the package that was current where the variable was declared. This is mostly useful as an alternative to the vars pragma, but also provides the opportunity to introduce typing and other attributes for such variables. See our in the perlfunc manpage.

Support for strings represented as a vector of ordinals

Literals of the form v1.2.3.4 are now parsed as a string composed of characters with the specified ordinals. This is an alternative, more readable way to construct (possibly Unicode) strings instead of interpolating characters, as in "\x{1}\x{2}\x{3}\x{4}". The leading v may be omitted if there are more than two ordinals, so 1.2.3 is parsed the same as v1.2.3.

Strings written in this form are also useful to represent version ``numbers''. It is easy to compare such version ``numbers'' (which are really just plain strings) using any of the usual string comparison operators eq, ne, lt, gt, etc., or perform bitwise string operations on them using |, &, etc.

In conjunction with the new $^V magic variable (which contains the perl version as a string), such literals can be used as a readable way to check if you're running a particular version of Perl:

    # this will parse in older versions of Perl also
    if ($^V and $^V gt v5.6.0) {
        # new features supported
    }

require and use also have some special magic to support such literals. They will be interpreted as a version rather than as a module name:

    require v5.6.0;             # croak if $^V lt v5.6.0
    use v5.6.0;                 # same, but croaks at compile-time

Alternatively, the v may be omitted if there is more than one dot:

    require 5.6.0;
    use 5.6.0;

Also, sprintf and printf support the Perl-specific format flag %v to print ordinals of characters in arbitrary strings:

    printf "v%vd", $^V;         # prints current version, such as "v5.5.650"
    printf "%*vX", ":", $addr;  # formats IPv6 address
    printf "%*vb", " ", $bits;  # displays bitstring

See Scalar value constructors in the perldata manpage for additional information.

Improved Perl version numbering system

Beginning with Perl version 5.6.0, the version number convention has been changed to a ``dotted integer'' scheme that is more commonly found in open source projects.

Maintenance versions of v5.6.0 will be released as v5.6.1, v5.6.2 etc. The next development series following v5.6.0 will be numbered v5.7.x, beginning with v5.7.0, and the next major production release following v5.6.0 will be v5.8.0.

The English module now sets $PERL_VERSION to $^V (a string value) rather than $] (a numeric value). (This is a potential incompatibility. Send us a report via perlbug if you are affected by this.)

The v1.2.3 syntax is also now legal in Perl. See Support for strings represented as a vector of ordinals for more on that.

To cope with the new versioning system's use of at least three significant digits for each version component, the method used for incrementing the subversion number has also changed slightly. We assume that versions older than v5.6.0 have been incrementing the subversion component in multiples of 10. Versions after v5.6.0 will increment them by 1. Thus, using the new notation, 5.005_03 is the ``same'' as v5.5.30, and the first maintenance version following v5.6.0 will be v5.6.1 (which should be read as being equivalent to a floating point value of 5.006_001 in the older format, stored in $]).

New syntax for declaring subroutine attributes

Formerly, if you wanted to mark a subroutine as being a method call or as requiring an automatic lock() when it is entered, you had to declare that with a use attrs pragma in the body of the subroutine. That can now be accomplished with declaration syntax, like this:

    sub mymethod : locked method ;
    ...
    sub mymethod : locked method {
        ...
    }

    sub othermethod :locked :method ;
    ...
    sub othermethod :locked :method {
        ...
    }

(Note how only the first : is mandatory, and whitespace surrounding the : is optional.)

AutoSplit.pm and SelfLoader.pm have been updated to keep the attributes with the stubs they provide. See the attributes manpage.

File and directory handles can be autovivified

Similar to how constructs such as $x->[0] autovivify a reference, handle constructors (open(), opendir(), pipe(), socketpair(), sysopen(), socket(), and accept()) now autovivify a file or directory handle if the handle passed to them is an uninitialized scalar variable. This allows the constructs such as open(my $fh, ...) and open(local $fh,...) to be used to create filehandles that will conveniently be closed automatically when the scope ends, provided there are no other references to them. This largely eliminates the need for typeglobs when opening filehandles that must be passed around, as in the following example:

    sub myopen {
        open my $fh, "@_"
             or die "Can't open '@_': $!";
        return $fh;
    }

    {
        my $f = myopen("</etc/motd");
        print <$f>;
        # $f implicitly closed here
    }

`open()` with more than two arguments

If open() is passed three arguments instead of two, the second argument is used as the mode and the third argument is taken to be the file name. This is primarily useful for protecting against unintended magic behavior of the traditional two-argument form. See open in the perlfunc manpage.

64-bit support

Any platform that has 64-bit integers either

        (1) natively as longs or ints
        (2) via special compiler flags
        (3) using long long or int64_t

is able to use ``quads'' (64-bit integers) as follows:

constants (decimal, hexadecimal, octal, binary) in the code
arguments to oct() and hex()
arguments to print(), printf() and sprintf() (flag prefixes ll, L, q)
printed as such
pack() and unpack() ``q'' and ``Q'' formats
in basic arithmetics: + - * / % (NOTE: operating close to the limits of the integer values may produce surprising results)
in bit arithmetics:

open() with more than two arguments

`open()` with more than two arguments