perl56delta.pod

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=head2 File globbing implemented internally

Perl now uses the File::Glob implementation of the glob() operator
automatically.  This avoids using an external csh process and the
problems associated with it.

    NOTE: This is currently an experimental feature.  Interfaces and
    implementation are subject to change.

=head2 Support for CHECK blocks

In addition to C<BEGIN>, C<INIT>, C<END>, C<DESTROY> and C<AUTOLOAD>,
subroutines named C<CHECK> are now special.  These are queued up during
compilation and behave similar to END blocks, except they are called at
the end of compilation rather than at the end of execution.  They cannot
be called directly.

=head2 POSIX character class syntax [: :] supported

For example to match alphabetic characters use /[[:alpha:]]/.
See L<perlre> for details.

=head2 Better pseudo-random number generator

In 5.005_0x and earlier, perl's rand() function used the C library
rand(3) function.  As of 5.005_52, Configure tests for drand48(),
random(), and rand() (in that order) and picks the first one it finds.

These changes should result in better random numbers from rand().

=head2 Improved C<qw//> operator

The C<qw//> operator is now evaluated at compile time into a true list
instead of being replaced with a run time call to C<split()>.  This
removes the confusing misbehaviour of C<qw//> in scalar context, which
had inherited that behaviour from split().

Thus:

    $foo = ($bar) = qw(a b c); print "$foo|$bar\n";

now correctly prints "3|a", instead of "2|a".

=head2 Better worst-case behavior of hashes

Small changes in the hashing algorithm have been implemented in
order to improve the distribution of lower order bits in the
hashed value.  This is expected to yield better performance on
keys that are repeated sequences.

=head2 pack() format 'Z' supported

The new format type 'Z' is useful for packing and unpacking null-terminated
strings.  See L<perlfunc/"pack">.

=head2 pack() format modifier '!' supported

The new format type modifier '!' is useful for packing and unpacking
native shorts, ints, and longs.  See L<perlfunc/"pack">.

=head2 pack() and unpack() support counted strings

The template character '/' can be used to specify a counted string
type to be packed or unpacked.  See L<perlfunc/"pack">.

=head2 Comments in pack() templates

The '#' character in a template introduces a comment up to
end of the line.  This facilitates documentation of pack()
templates.

=head2 Weak references

In previous versions of Perl, you couldn't cache objects so as
to allow them to be deleted if the last reference from outside 
the cache is deleted.  The reference in the cache would hold a
reference count on the object and the objects would never be
destroyed.

Another familiar problem is with circular references.  When an
object references itself, its reference count would never go
down to zero, and it would not get destroyed until the program
is about to exit.

Weak references solve this by allowing you to "weaken" any
reference, that is, make it not count towards the reference count.
When the last non-weak reference to an object is deleted, the object
is destroyed and all the weak references to the object are
automatically undef-ed.

To use this feature, you need the Devel::WeakRef package from CPAN, which
contains additional documentation.

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

=head2 Binary numbers supported

Binary numbers are now supported as literals, in s?printf formats, and
C<oct()>:

    $answer = 0b101010;
    printf "The answer is: %b\n", oct("0b101010");

=head2 Lvalue subroutines

Subroutines can now return modifiable lvalues.
See L<perlsub/"Lvalue subroutines">.

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

=head2 Some arrows may be omitted in calls through references

Perl now allows the arrow to be omitted in many constructs
involving subroutine calls through references.  For example,
C<< $foo[10]->('foo') >> may now be written C<$foo[10]('foo')>.
This is rather similar to how the arrow may be omitted from
C<< $foo[10]->{'foo'} >>.  Note however, that the arrow is still
required for C<< foo(10)->('bar') >>.

=head2 Boolean assignment operators are legal lvalues

Constructs such as C<($a ||= 2) += 1> are now allowed.

=head2 exists() is supported on subroutine names

The exists() builtin now works on subroutine names.  A subroutine
is considered to exist if it has been declared (even if implicitly).
See L<perlfunc/exists> for examples.

=head2 exists() and delete() are supported on array elements

The exists() and delete() builtins now work on simple arrays as well.
The behavior is similar to that on hash elements.

exists() can be used to check whether an array element has been
initialized.  This avoids autovivifying array elements that don't exist.
If the array is tied, the EXISTS() method in the corresponding tied
package will be invoked.

delete() may be used to remove an element from the array and return
it.  The array element at that position returns to its uninitialized
state, so that testing for the same element with exists() will return
false.  If the element happens to be the one at the end, the size of
the array also shrinks up to the highest element that tests true for
exists(), or 0 if none such is found.  If the array is tied, the DELETE() 
method in the corresponding tied package will be invoked.

See L<perlfunc/exists> and L<perlfunc/delete> for examples.

=head2 Pseudo-hashes work better

Dereferencing some types of reference values in a pseudo-hash,
such as C<< $ph->{foo}[1] >>, was accidentally disallowed.  This has
been corrected.

When applied to a pseudo-hash element, exists() now reports whether
the specified value exists, not merely if the key is valid.

delete() now works on pseudo-hashes.  When given a pseudo-hash element
or slice it deletes the values corresponding to the keys (but not the keys
themselves).  See L<perlref/"Pseudo-hashes: Using an array as a hash">.

Pseudo-hash slices with constant keys are now optimized to array lookups
at compile-time.

List assignments to pseudo-hash slices are now supported.

The C<fields> pragma now provides ways to create pseudo-hashes, via
fields::new() and fields::phash().  See L<fields>.

    NOTE: The pseudo-hash data type continues to be experimental.
    Limiting oneself to the interface elements provided by the
    fields pragma will provide protection from any future changes.

=head2 Automatic flushing of output buffers

fork(), exec(), system(), qx//, and pipe open()s now flush buffers
of all files opened for output when the operation was attempted.  This
mostly eliminates confusing buffering mishaps suffered by users unaware
of how Perl internally handles I/O.

This is not supported on some platforms like Solaris where a suitably
correct implementation of fflush(NULL) isn't available.

=head2 Better diagnostics on meaningless filehandle operations

Constructs such as C<< open(<FH>) >> and C<< close(<FH>) >>
are compile time errors.  Attempting to read from filehandles that
were opened only for writing will now produce warnings (just as
writing to read-only filehandles does).

=head2 Where possible, buffered data discarded from duped input filehandle

C<< open(NEW, "<&OLD") >> now attempts to discard any data that
was previously read and buffered in C<OLD> before duping the handle.
On platforms where doing this is allowed, the next read operation
on C<NEW> will return the same data as the corresponding operation
on C<OLD>.  Formerly, it would have returned the data from the start
of the following disk block instead.

=head2 eof() has the same old magic as <>

C<eof()> would return true if no attempt to read from C<< <> >> had
yet been made.  C<eof()> has been changed to have a little magic of its
own, it now opens the C<< <> >> files.

=head2 binmode() can be used to set :crlf and :raw modes

binmode() now accepts a second argument that specifies a discipline
for the handle in question.  The two pseudo-disciplines ":raw" and
":crlf" are currently supported on DOS-derivative platforms.
See L<perlfunc/"binmode"> and L<open>.

=head2 C<-T> filetest recognizes UTF-8 encoded files as "text"

The algorithm used for the C<-T> filetest has been enhanced to
correctly identify UTF-8 content as "text".

=head2 system(), backticks and pipe open now reflect exec() failure

On Unix and similar platforms, system(), qx() and open(FOO, "cmd |")
etc., are implemented via fork() and exec().  When the underlying
exec() fails, earlier versions did not report the error properly,
since the exec() happened to be in a different process.

The child process now communicates with the parent about the
error in launching the external command, which allows these
constructs to return with their usual error value and set $!.

=head2 Improved diagnostics

Line numbers are no longer suppressed (under most likely circumstances)
during the global destruction phase.

Diagnostics emitted from code running in threads other than the main
thread are now accompanied by the thread ID.

Embedded null characters in diagnostics now actually show up.  They
used to truncate the message in prior versions.

$foo::a and $foo::b are now exempt from "possible typo" warnings only
if sort() is encountered in package C<foo>.

Unrecognized alphabetic escapes encountered when parsing quote
constructs now generate a warning, since they may take on new
semantics in later versions of Perl.

Many diagnostics now report the internal operation in which the warning
was provoked, like so:

    Use of uninitialized value in concatenation (.) at (eval 1) line 1.
    Use of uninitialized value in print at (eval 1) line 1.

Diagnostics  that occur within eval may also report the file and line
number where the eval is located, in addition to the eval sequence
number and the line number within the evaluated text itself.  For
example:

    Not enough arguments for scalar at (eval 4)[newlib/perl5db.pl:1411] line 2, at EOF

=head2 Diagnostics follow STDERR

Diagnostic output now goes to whichever file the C<STDERR> handle
is pointing at, instead of always going to the underlying C runtime
library's C<stderr>.

=head2 More consistent close-on-exec behavior

On systems that support a close-on-exec flag on filehandles, the
flag is now set for any handles created by pipe(), socketpair(),
socket(), and accept(), if that is warranted by the value of $^F
that may be in effect.  Earlier versions neglected to set the flag
for handles created with these operators.  See L<perlfunc/pipe>,
L<perlfunc/socketpair>, L<perlfunc/socket>, L<perlfunc/accept>,
and L<perlvar/$^F>.

=head2 syswrite() ease-of-use

The length argument of C<syswrite()> has become optional.

=head2 Better syntax checks on parenthesized unary operators

Expressions such as:

    print defined(&foo,&bar,&baz);
    print uc("foo","bar","baz");
    undef($foo,&bar);

used to be accidentally allowed in earlier versions, and produced
unpredictable behaviour.  Some produced ancillary warnings
when used in this way; others silently did the wrong thing.

The parenthesized forms of most unary operators that expect a single
argument now ensure that they are not called with more than one
argument, making the cases shown above syntax errors.  The usual
behaviour of:

    print defined &foo, &bar, &baz;
    print uc "foo", "bar", "baz";
    undef $foo, &bar;

remains unchanged.  See L<perlop>.

=head2 Bit operators support full native integer width

The bit operators (& | ^ ~ << >>) now operate on the full native
integral width (the exact size of which is available in $Config{ivsize}).
For example, if your platform is either natively 64-bit or if Perl
has been configured to use 64-bit integers, these operations apply
to 8 bytes (as opposed to 4 bytes on 32-bit platforms).
For portability, be sure to mask off the excess bits in the result of
unary C<~>, e.g., C<~$x & 0xffffffff>.

=head2 Improved security features

More potentially unsafe operations taint their results for improved
security.

The C<passwd> and C<shell> fields returned by the getpwent(), getpwnam(),
and getpwuid() are now tainted, because the user can affect their own
encrypted password and login shell.

The variable modified by shmread(), and messages returned by msgrcv()
(and its object-oriented interface IPC::SysV::Msg::rcv) are also tainted,
because other untrusted processes can modify messages and shared memory
segments for their own nefarious purposes.

=head2 More functional bareword prototype (*)

Bareword prototypes have been rationalized to enable them to be used
to override builtins that accept barewords and interpret them in
a special way, such as C<require> or C<do>.

Arguments prototyped as C<*> will now be visible within the subroutine
as either a simple scalar or as a reference to a typeglob.
See L<perlsub/Prototypes>.

=head2 C<require> and C<do> may be overridden

C<require> and C<do 'file'> operations may be overridden locally
by importing subroutines of the same name into the current package 
(or globally by importing them into the CORE::GLOBAL:: namespace).
Overriding C<require> will also affect C<use>, provided the override
is visible at compile-time.
See L<perlsub/"Overriding Built-in Functions">.

=head2 $^X variables may now have names longer than one character

Formerly, $^X was synonymous with ${"\cX"}, but $^XY was a syntax
error.  Now variable names that begin with a control character may be
arbitrarily long.  However, for compatibility reasons, these variables
I<must> be written with explicit braces, as C<${^XY}> for example.
C<${^XYZ}> is synonymous with ${"\cXYZ"}.  Variable names with more
than one control character, such as C<${^XY^Z}>, are illegal.

The old syntax has not changed.  As before, `^X' may be either a
literal control-X character or the two-character sequence `caret' plus
`X'.  When braces are omitted, the variable name stops after the
control character.  Thus C<"$^XYZ"> continues to be synonymous with
C<$^X . "YZ"> as before.

As before, lexical variables may not have names beginning with control
characters.  As before, variables whose names begin with a control
character are always forced to be in package `main'.  All such variables
are reserved for future extensions, except those that begin with