A relatively simple obfuscation for a relatively simple substitution cipher. This either takes a file as a parameter to encrypt in atbash, or provided no input encrypt's it's __DATA__.

#!/usr/bin/perl

open DATA,$ARGV[0]if$ARGV[0];$ ,=65;@ .=map$_,reverse chr$ ,..pack'c',$ .;$.=$,
+25;print join'',map{join'',map{$ *=ord>ord uc z?push(@%,/./)&&uc:push(@%,0)&&$
_;$ ^=$ *=~/[chr$,-chr$.]/?$.[(ord$*)-$,]:$*;$%[$;++]?lc$^:$^}split q,,,}

__DATA__
This is an implementation of an atbash (or 'zgyzhs') cipher, which encodes
strings by replacing 'a' with 'z', 'b' with 'y', etc.  This is my test string.
This string includes CAPital letters, spaces, punctuation, and newlines.  This
tests EVERY capability of this implementation.

$,=42;for(34,0,-3,9,-11,11,-17,7,-5){$*.=pack'c'=>$,+=$_}for(reverse split//=>$*
){$%++?$ %%2?push@C,$_,$":push@c,$_,$":(push@C,$_,$")&&push@c,$"}$C[$#C]=$/;($#C
>$#c)?($ c=\@C)&&($ C=\@c):($ c=\@c)&&($C=\@C);$%=$|;for(@$c){print$_^$$C[$%++]}