You could also use /etc/mail/aliases directly, similar to the way many mailing list applications do, such as Mailman, majordomo and the like. The syntax is easy:

user@domain.com:          "|/path/to/perl/script.pl"
# Make sure to run 'newaliases' to regenerate
[download]

There's also Chip Salzenberg's deliver program documented here. There are many native ways which don't rely on necessarily installing core perl modules such as Mail::Audit (tutorial here) and friends. When working on boxes where installing externally available perl modules is not an option (i.e. for paranoid sysadmins), this may be a better alternative (TMTOWTDI).