=item MS-DOS
Create a batch file to run your program, and codify it in
-C<ALTERNATIVE_SHEBANG> (see the F<dosish.h> file in the source
+C<ALTERNATE_SHEBANG> (see the F<dosish.h> file in the source
distribution for more information).
=item Win95/NT
=item PERL5LIB
-A colon-separated list of directories in which to look for Perl library
+A list of directories in which to look for Perl library
files before looking in the standard library and the current
directory. Any architecture-specific directories under the specified
locations are automatically included if they exist. If PERL5LIB is not
-defined, PERLLIB is used.
+defined, PERLLIB is used. Directories are separated (like in PATH) by
+a colon on unixish platforms and by a semicolon on Windows (the proper
+path separator being given by the command C<perl -V:path_sep>).
When running taint checks (either because the program was running setuid
or setgid, or the B<-T> switch was used), neither variable is used.
=item :crlf
-A layer that implements DOS/Windows like CRLF line endings.
-On read converts pairs of CR,LF to a single "\n" newline character.
-On write converts each "\n" to a CR,LF pair.
+A layer that implements DOS/Windows like CRLF line endings. On read
+converts pairs of CR,LF to a single "\n" newline character. On write
+converts each "\n" to a CR,LF pair. Note that this layer likes to be
+one of its kind: it silently ignores attempts to be pushed into the
+layer stack more than once.
+
+(Gory details follow) To be more exact what happens is this: after
+pushing itself to the stack, the C<:crlf> layer checks all the layers
+below itself to find the first layer that is capable of being a CRLF
+layer but is not yet enabled to be a CRLF layer. If it finds such a
+layer, it enables the CRLFness of that other deeper layer, and then
+pops itself off the stack. If not, fine, use the one we just pushed.
+
+The end result is that a C<:crlf> means "please enable the first CRLF
+layer you can find, and if you can't find one, here would be a good
+spot to place a new one."
+
Based on the C<:perlio> layer.
=item :mmap
=item PERLLIB
-A colon-separated list of directories in which to look for Perl library
+A list of directories in which to look for Perl library
files before looking in the standard library and the current directory.
If PERL5LIB is defined, PERLLIB is not used.
=item PERL_HASH_SEED
-(Since Perl 5.8.1.)
-
-Used to randomise Perl's internal hash function. To emulate the
-pre-5.8.1 behaviour, set to an integer (zero means exactly the same
-order as 5.8.0). "Pre-5.8.1" means, among other things, that hash
-keys will be ordered the same between different runs of Perl.
+(Since Perl 5.8.1.) Used to randomise Perl's internal hash function.
+To emulate the pre-5.8.1 behaviour, set to an integer (zero means
+exactly the same order as 5.8.0). "Pre-5.8.1" means, among other
+things, that hash keys will be ordered the same between different runs
+of Perl.
The default behaviour is to randomise unless the PERL_HASH_SEED is set.
-If Perl has been compiled with the -DUSE_HASH_SEED_EXPLICIT the default
+If Perl has been compiled with C<-DUSE_HASH_SEED_EXPLICIT>, the default
behaviour is B<not> to randomise unless the PERL_HASH_SEED is set.
If PERL_HASH_SEED is unset or set to a non-numeric string, Perl uses
the pseudorandom seed supplied by the operating system and libraries.
-If unset, each different run of Perl will have different ordering of
-the outputs of keys(), values, and each().
+This means that each different run of Perl will have a different
+ordering of the results of keys(), values(), and each().
+
+B<Please note that the hash seed is sensitive information>. Hashes are
+randomized to protect against local and remote attacks against Perl
+code. By manually setting a seed this protection may be partially or
+completely lost.
-See L<perlsec/"Algorithmic Complexity Attacks"> for more information.
+See L<perlsec/"Algorithmic Complexity Attacks"> and
+L</PERL_HASH_SEED_DEBUG> for more information.
=item PERL_HASH_SEED_DEBUG
-Set to (anything) to display the value of the hash seed.
+(Since Perl 5.8.1.) Set to one to display (to STDERR) the value of
+the hash seed at the beginning of execution. This, combined with
+L</PERL_HASH_SEED> is intended to aid in debugging nondeterministic
+behavior caused by hash randomization.
+
+B<Note that the hash seed is sensitive information>: by knowing it one
+can craft a denial-of-service attack against Perl code, even remotely,
+see L<perlsec/"Algorithmic Complexity Attacks"> for more information.
+B<Do not disclose the hash seed> to people who don't need to know it.
+See also hash_seed() of L<Hash::Util>.
=item PERL_ROOT (specific to the VMS port)
In Perls 5.8.1 and later. If set to C<unsafe> the pre-Perl-5.8.0
signals behaviour (immediate but unsafe) is restored. If set to
-C<safe> the safe (or deferred) signals are used. See L<perlipc>.
+C<safe> the safe (or deferred) signals are used.
+See L<perlipc/"Deferred Signals (Safe signals)">.
=item PERL_UNICODE
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