It's not obvious which format this module supports. No mention of 'qmail' nor 'maildir'.
http://search.cpan.org/~markov/Mail-Box-2.120/lib/Mail/Box/Manager.pod
It has functions to move between folders, which sounds like maildir, but then it also hints that it encapsulates Mail::Box::Mbox, which sounds like the single file mbox format.
Here is a description of maildir format: http://wiki.dovecot.org/MailboxFormat/Maildir, and description of both: http://www.postfix.org/virtual.8.html
http://search.cpan.org/~markov/Mail-Box-2.120/lib/Mail/Box-Overview.pod