rot13 == add 13 to the value of a character.

I've only ever used it on alpha type data streams, but I assume you could do something similar on numbers as well. Simple rot13 key gen below. The value in the top row becomes the value in the bottom row, and vice versa

$f = 'a';

for ( 0 .. 25 ) {
   print "$f ";
   print "\n" if $_ == 12;
   $f++;
}
print "\n";
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