the same 3-character $reqText produces 50-character encrypted output on the new server.

Are you sure it's not 48 characters? Newer versions of Crypt::CBC transmit the string "RandomIV" followed by eight random characters, then the first eight-character encrypted block. That comes out to 48 hex digits. Older versions use an IV precomputed from the key, and start transmitting ciphertext immediately.

The versions of the CBC and IDEA modules vary on each server but only very slightly.

Specifically what version of Crypt::CBC do you have? The RandomIV thing was added in 1.21.

Unfortunately, I don't see any way to get old versions of Crypt::CBC to play nice with new versions without a lot of pain and agony. Is it workable for you to upgrade Crypt::CBC on one or both machines?

    $cipher = new Crypt::CBC('$key','IDEA');
[download]

I'm going to assume your actual code doesn't have $key in single quotes... right? I hope?