I think the RC4 algorithm is generally considered a good compromise between speed and security. There are several CPAN modules implementing it, e.g. Crypt::RC4::XS.

Update: More generally, if speed is of paramount importance (and you feel like playing with this), find a pseudo random number generator that's extremely fast but still produces reasonable quality randomness, i.e. 'random' enough to match your idea of what isn't "extremely simple to decrypt".  Then, to encrypt, bitwise XOR the data with the pseudo random number sequence. To decrypt, just do the same XORing once again with the same random sequence, i.e. using the same PRNG seed, which could be derived from a passphrase, together with some 'salt' (the latter is some additional random data generated anew for every encrypt/decrypt cycle, and stored with the message).

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