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.