There is a contention issue here. This file is large. What if the mail program writes to the mailbox before the program is finished processing?

At first I thought, no problem, just use flock. But then I read what Camel III says about flock:

Despite the suggestive sound of "exclusive", processes aren't required to obey locks on files. That is, flock only implements advisory locking, which means that locking a file does not stop another process from reading or even writing the file.

Contention is a major problem, because the time to process a 30MB file is not trivial, and in the meantime more mail is on its way at any moment. I question whether a simple program can solve the problem, so maybe Mail::Folder::Mbox or Mail::Box::Mbox are needed after all.

It's the classic Hubris (I can write this myself) vs. Laziness (I don't want to reinvent the wheel) problem. I believe that Laziness should trump Hubris, and in a real world setting it would thus be better to use the CPAN modules.