Well, it depends on your processor. For example, I have used PayPal Payflow Pro and have had great success with it. And there are quite a few Business::PayPal modules on CPAN.

I would start with your processor and see if you can find anything on CPAN. If you use some obscure payment method you might be stuck writing your own. Either way, the scheme I mentioned earlier will help... just don't get your processor entangled into your web app or you may be doing some head-banging later on.

Looking back (it has been about a year since I have used Payflow) but it was a little quarky then. Hopefully, it is better developed now (on the PayPal side). Once I got everything sorted out during development it ran great in production.