Here's something I whipped up for fun:

#!/usr/bin/env perl -pi.orig
use warnings;
use strict;
BEGIN {
    @ARGV = $0;
    while (<>) {
        do { print; last; } if /^__DATA__$/;
    } continue {
        print;
    }
}
tr/A-Z/N-ZA-M/;
tr/a-z/n-za-m/;
__DATA__
Guvf vf n cebtenz gung zbqvsvrf vgfrys va cynpr.
Vg ebg13 rapbqrf vgf bja __QNGN__ frpgvba jura lbh eha vg.
Nalguvat orsber gur __QNGN__ frpgvba vf yrsg hazbqvsvrq.
Lbh jvyy frr gur sbyybjvat qvntabfgvp reebe, juvpu lbh pna vtaber:

    -v hfrq jvgu ab svyranzrf ba gur pbzznaq yvar, ernqvat sebz FGQVA.
[download]