Thunderbird extensions are written in Javascript, not in Perl.

Personally, I manage my IMAP accounts by using Net::IMAPClient and use that as a spam filter. Together with MIME::Parser, you can also extract and open attachments immediately. But to integrate that closely with Thunderbird, you will need to program in Javascript. Also, automatically opening attachments is a vast security hole which will allow executables to automatically run under your security context, which is a bad idea.