Have you tried re-opening the file, or alternatively just seek($mail_fh,0,0) to re-position the file pointer to the beginning of the file?

I didn't actually try it (because I don't have those modules installed), but the following statement from the documentation of Mail::Internet's new() method, i.e. "the new object will be initialized with headers and body" makes me suspect that the module might have already read the entire file (not just the headers), so the file pointer would need to be re-positioned for another read of the file (by MIME::Parser)...