JWZ has a nice overview on the issues of different mbox formats. Also see the Wikipedia article.
both Email::Folder::Mbox and Mail::Box::Mbox have been tried, but found to have issues parsing some of the archives this project was being developped for. Chj::Parse::Mbox with a simple approach of matching \nFrom as separator worked better on the archives in question; this will probably fail on those mbox files that JWZ mentions relying on Content-Length, though, if so, one of the formerly-mentioned parsers may have to be tried again (for this, adapt lib/Chj/Ml2json/Mbox.pm so that it outputs a stream instead of an iterator, and add some commandline option for the choice). Also, Mail::Box::Mbox creates lock files, if those are conformant to some generally supported spec then that may be useful; Chj::Parse::Mbox does not attempt to do any locking, which will not matter as long as the files in question are not being modified once put in place, of course (the usual unixy way of writing contents to a temp file then renaming it would make this safe, still; appending to the existing file would not be safe).