See for example MIME::Lite.

There may be better ways to do it, but I decided to pull out a script I used when testing a mail server and modify it to do what you described, within the restriction of trying not to use anything not listed as a core module. (Mostly I succeeded-the only exception would be if you require SMTP authentication, where you may need Authen::SASL and MIME::Base64.)

The script creates valid headers, a plain text component with a list of files attached, and attaches each file as a UUEncoded attachment. It also allows you to specify the outgoing mail server/port, your credentials for SMTP authentication, sender, subject, multiple recipient addresses, a test mode to see what the output will look like, and an SMTP debug option to allow you to watch the SMTP transaction.

Hope that helps.