Lots of ways to do this, here's the ones I remember off the top of my head, pick one you like:

• Look for files in a directory/drop files in a directory (good when data is in obvious chunks like transactions, and/or you're in a hurry.)
• Create a pipe. One process writes to it, the other reads. If you need bi-directional, two pipes. (This is classic *nix.)
• If there is a future possibility of the two processes being on different machines, consider using a socket.
• I haven't used SOAP::Lite, but a couple other posters mention it, so it's probably worth checking, especially if your project needs more buzzwords for PHB approval.

Also, if there is a future possibility of needing more than one instance of either end of the protocol, keep that in mind in picking a solution.
--
Spring: Forces, Coiled Again!