I've done something like this in the past and I used MIME::Parser and MIME::Lite for it. I used a SMTP server for the sending, but for various stupid reasons I needed to use VB in Outlook to handle the receipt function. Thus I can't recommend any Perl solution for that aspect. (Although i see Mail::Box mentioned favourably quite often, so perhaps thats a good place to look).

Just so you know you'll need to be very careful about how you rebundle the mail. By blindly appending content to the end of signed or encrypted mail you can make the mail look forged or break the encryption. IE, its critical that you analyse the MIME container structure to make sure you dont mangle anything.

Good luck.