On the CPAN is a list of Modules named Business::OnlinePayment

They appear to be interfaces to various transaction brokers, so you'd just have to set up an account with one and you're off to the races for accepting transactions. For more full featured store management software I would look into something like Interchange as mentioned by another monk.

It really looks like a great place to start.

Best of luck.

--
Clayton aka "Tex"