While this can be done in Perl (see, for example crypt), this is not entirely a Perl question, at least the way you've phrased it.

So, is this a programming exercise? If so, there are many encryption/decryption algorithms. Probably the easiest is xor-ing the plain text, character by character, with a key. There are also transposition ciphers, which scramble word order according to a key.

If this isn't an exercise, you could try gpg, presuming it's legal for you to download and use it. There's also an associated Perl module, gpg.