My first clue is to explain ACA in the post. It's probably not very helpful; just a nudge in a direction. Here's the link:

My next clue is somethign most here could do themselves, but I just learned it this week (from reading PerlMonks!). Here's the perl -MO=Deparse output (this is without eyepopslikeamosquito's reduction):

My next clue is to comment my variables. This is a big leap and probably a give away. I wish I had better intermediate clues for ya.

# @ca - code array
# @kp - key phrase
# $c  - caesar shift (big hint here)
# %p2 - short for p2C - plaintext to CIPHERTEXT hash
# %C2 - short for C2p - CIPHERTEXT to plaintext hash
# $k  - key (one letter of the key alphabet)
# $t  - text
# $h  - hash reference
# $r  - result
[download]

ok, here's what I'm doing: