For completely unbreakable encryption with core Perl, use the binary XOR operator ("^") and a one-time pad. If an OTP is unsuitable for your purposes (and it almost certainly is, because there are few practical cases in which an OTP can be used sensibly and correctly), use one of the Crypt modules, as suggested by others above. Don't be afraid of CPAN modules, they are a part of Perl, even if they are usually not distributed together with the binary.