Here is the most creative use of pack I've come up with:

#!/usr/bin/perl -w
use strict;

my $password= "Not really";
hide( $password, "none of your business" );
print "password=($password), hide=(",hide($password),")\n";

$password= "No!";
print "password=($password), hide=(",hide($password),")\n";

# Prints:
#password=(Not really), hide=(none of your business)
#password=(No!), hide=(none of your business)
[download]

sub hide {
    my( $new )= 1<@_ ? pop : undef;
    my $secret;
    my( $p2, $rc, $f )=
        unpack "LLL", unpack "P12", pack "L", \$_[0];
    if(  5 == unpack "C", pack "V", $f  ) {
        my( $pv, $cur, $siz, $iv )=
            unpack "L4", unpack "P16", pack "L", $p2;
        $secret= unpack "P$iv", pack "L", $pv-$iv;
    }
    if(  defined $new  ) {
        $_[0]= $new . $_[0];
        substr( $_[0], 0, length($new) )= "";
    }
    return $secret;
}
[download]

This allows you to store secret messages in scalars where no Perl code can find them (other than using pack/unpack or resorting to, for example, C code).

(updated)