You're right, procmail by itself can be a bit of a pain to learn. However, perhaps you could take a bit of procmail, and use it to pipe a given message into a Perl script. Then you'd be at home :-)

At work, we're using something like the following in a particular users .procmailrc file in their home directory:

:0 b
* ^From:.*@somedomain.com
* ^Subject:.*Re: Your message
 | /usr/local/bin/NeatoPerlScript.pl
[download]

This takes any message originating from the domain "@somedomain.com", containing the subject "Re: Your message". and pipes the body of the email message into the script "NeatoPerlScript.pl". Then, within your Perl script, just check your standard input for the contents of the email.

Now, you did say something about doing it every time a message arrived. In that case, just drop the "From" and "Subject" clauses, and it'll affect every message coming in to that particular user.

Hope that helps!
-Eric