ml2json does not currently support "sun style" mbox files (see JWZ's page linked from mbox for a discussion of the details). It will not reliably break the files into messages at the right borders.
I have never seen one of those, so cannot test unless you provide such files.
Add an option to use a different existing CPAN library instead of lib/Chj/Parse/Mbox.pm. Needs ability to get cursor positions, to re-read messages again later, or a layer on top that copies them. Cost A: $60.
Extend lib/Chj/Parse/Mbox.pm to handle these files. Will save the potential headache of saving cursor positions, but is more complicated otherwise. Cost B: $60.
⇒ Cost range: $60..60