What level of security is needed? For strong security there is Crypt::GPG. For weak obfuscation from casually prying eyes, there's ROT13, which could be written easily as "$string =~ tr/A-Za-z/N-ZA-Mn-za-m/;". Applied once, you rot-13 the standard English alphabet so that it can't be casually read. Applied a second time, and you undo the previous rotation.

What is your ultimate objective, bullet proof security, mild obfuscation, ASCII encoding of binary data, or what?