While I haven't used Crypt::DES_PP, ECB mode is trivial. You just encrypt/decrypt the data, 8 bytes at a time, using the same key for every encrypt/decrypt operation.

Since DES (aka Lucifer) was designed to work on early 1970s microprocesses (or even just on an array of logic gates), a pure Perl implementation should not have bad performance, so go ahead and try it.

If you are curious about the various modes of encryption, see http://en.wikipedia.org/wiki/Block_cipher_mode_of_operation.