in reply to Mail::Box::Mbox adds a strange boundary when read email

Hrm, that's pretty standard stuff....

Content-Type: multipart/related; boundary="_----------=_11640761411305 +30"

That is a MIME header that says your document is broken up into peices, divided by your "strange line." Mail::Box definitely knows how to read MIME messages, so I suspect you're not using the module to its full potential. It's possible the mail is damaged in some way. It looks acceptable to my tired eyes though.

-Paul