This question is answered in the IAQ, under the question "How do I decrypt a string that I encrypted with crypt()?" (since crypt generates "UNIX style passwords", usually). To quote their answer:

        sub decrypt {
          my $c = shift;
          my @c = (0) x 8;
          for (;;) {
            my $i = 0;
            my $s = join '', map chr, @c;
            return $s if crypt($s, $c) eq $c;
            $c[$i]=0, $i++ while $c[$i] == 255;
            return undef if $i > 7;
            $c[$i]++;
          }
        }
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(For the humor impaired, the IAQ is not serious; quoth crypt's docs: "Note that "crypt" is intended to be a one-way function... There is no (known) corresponding decrypt function." This functions attempts to brute force the crypt'd string and thus is extremely slow.)