#!/usr/bin/perl
use yX;
...

##</code><code>##

$ cat yX.pm
package yX;

use Filter::Simple sub {
   tr/n-za-mN-ZA-M/a-zA-Z/;
};

1;

##</code><code>##

$ cat a.pl
#!/usr/bin/perl
use yX;
cevag("Uryyb, Jbeyq!\a");

##</code><code>##

$ a.pl
Hello, World!