The only legitimate reason for doing this would be to return to the user their password if they've forgotten it. Because it's impossible, you are better off creating a new random password and emailing them.

Nefarious people don't nessarily have to brute force the actual password - they just need to find a hashing collision. i.e. if password "foo" and "bar" both hash to "abc", then they are identical as far as the password check goes. This may not seem like much of a distinction (especially as the hash is often longer, and the input salted), but it means if the hash algorithm is weak, then it can be exploited. I believe there has been some preliminary weakness in SHA (some researchers suggest that it could be cracked in, say, 75 years rather than 100 years), but overall they appear strong.

And if you allow passwords of length less than 1..

sub decrypt {
  my $crypted = shift;
  my $str = "";
  foreach my $s1 (0 .. 255) {
    my $str = $str . chr $s1;
    if ($cryped eq crypt($str, $crypted)) {
                    return $str
    }
    foreach my $s2 (0 .. 255) {
      my $str = $str . chr $s2;
      if ($cryped eq crypt($str, $crypted)) {
                    return $str
      }
      foreach my $s3 (0 .. 255) {
        my $str = $str . chr $3;
        if ($cryped eq crypt($str, $crypted)) {
                    return $str
        }
        foreach my $s4 (0 .. 255) {
          my $str = $str . chr $4;
          if ($cryped eq crypt($str, $crypted)) {
                    return $str
          }
          foreach my $s5 (0 .. 255) {
            my $str = $str . chr $5;
            if ($cryped eq crypt($str, $crypted)) {
                    return $str
            }
            foreach my $s6 (0 .. 255) {
              my $str = $str . chr $6;
              if ($cryped eq crypt($str, $crypted)) {
                    return $str
              }
              foreach my $s7 (0 .. 255) {
                my $str = $str . chr $7;
                if ($cryped eq crypt($str, $crypted)) {
                    return $str
                }
                foreach my $s8 (0 .. 255) {
                  my $str = $str . $chr $8;
                  if ($cryped eq crypt($str, $crypted)) {
                    return $str
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}
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