#!/usr/bin/perl -w

$c = '

$#ARGV + 1 or print "$0 <key>\n" and die;

($p = shift) =~ s/[^a-zA-Z]//g;
@q = map {$_ < 91 ? (91 - $_) % 26 : $_ - 97} unpack "C*", $p || "a";

for($b = 26; $b--;)
{
   $d{$_}{$b} = $_ foreach 0..255;
   for($o = 26; $o--; map {$d{$_ + $o}{$b} = $_ + ($o + $b) % 26} (97, 65)) {}
}

for($o = 0; $i = <>;)
{
   foreach (unpack "C*", $i)
   {
      ($c = chr$d{$_}{$q[$o]}) =~ /[a-zA-Z]/ && $o++;
      $o %= $#q + 1;
      print $c;
   }
}

';

'code' =~ /code(?{eval $c})/;