How does your ISP require you to authenticate -- is it SMTP AUTH or POP-before-SMTP or something even more exotic? If it's POP-before-SMTP, use Net::POP3 to log into the POP server once per session, before submitting any outgoing mail (you shouldn't need to actually download any messages).

If it's SMTP AUTH it looks like you might be out of luck, unless you feel like adding this functionality yourself (which actually doesn't look that difficult).