A few non-Perl, Yahoo!-specific things to consider:

• Sign up in the POP Access and Forwarding section of Options.
• 'Need Help?' for POP3 access tells you what you need to use this access (server names, etc.)
• You must authenticate when you send SMTP messages via their service.

As for Perl, TIMTOWTDI. The combination of Mail::Sender (for the SMTP) and Mail::POP3Client (for the authentication) may work with Yahoo! mail without the need for SendMail. Both are available on ActiveState's PPM Package Repository.