#!/usr/bin/perl
@l=(a..z);$0=~m/(\d+)/;$z=shift||$1||13;$z%=26;$s=$z-1;$c=-1;while($c!=
$#l){$s++;$c++;if($s>$#l){$s=0}$r[$c]=$l[$s]}$f="@l".uc("@l");$t="@r".
uc("@r");while(<>){eval "y/$f/$t/";print} # rot13,14...whatever in 3.