It doesn't matter how long your "pad" is, if you cycle through it in a predictable fashion. If the message is longer than the key, you will reuse key bytes. If you reuse key bytes, you're a sitting duck. 256 bytes of encrypted English text would be easy to break in a matter of minutes. Half that or less would probably be enough.
When you're doing crypto, don't try to get clever. Use a real cipher module like Crypt::Rijndael or Crypt::IDEA or whatever. There was an article on crypto modules on perl.com recently.
If for some reason you can't use one of those, there are secure ways to use hash functions like MD5 for encryption. You have to be very careful, though, because that's not what they were designed to do. For details, see <cite>Applied Cryptography</cite>.
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