can MHonArc (or another utility) retreive mail from a POP3 server and then
No. What you have to do is use a POP3 client to download the mail
from the mail server and then pipe the messages to mhonarc. If
looking for a Perl solution, there is a POP3 module available at
CPAN. With it, you should be able to automate the task.
Another alternative is to use fetchmail to connect to the POP3
server and pull the messages over to local disk. This gives you
more pop support (APOP, KPOP, etc.) as well as the ability
to pipe messages through an application like mhonarc (or procmail,
which could do funky stuff and then call mhonarc), all in
one fell swoop.
Much more secure and, depending on someone's coding abilities,
probably more robust and faster.