I don't know what exactly you want to do (if it's just users having more than one email address, that's trivial in sendmail, just set up an alias), but sendmail isn't modular in the sense that you can replace certain parts of it with code you wrote. And I don't know any MTA that does.

However, if it's just some kind of post-processing (which would include any redirection) of incoming email, that's fairly trivial with MTAs like sendmail, qmail or postfix. But that's not a Perl issue anymore, and you're better off consulting the various manual pages.

Abigail