A quick Google search revealed this project on FreshMeat with a Perl/MySQL shopping cart. You might want to check that out to see what was done, and how it was done, just to hopefully save you the pitfalls if you're doing this yourself from scratch.

There is another one I've seen on the web that is open source that I was playing with a while back but I can't find it at the moment. That one clocked in at about 3000 lines of Perl code and I was going to have to make modifications to make it work for me.

Do realize that with any shopping cart, if you're doing US sales, you have multiple sales tax rates to deal within the country and sometimes within states. California itself has sales tax rates by county. You're also going to have to probably figure out your own shipping charges routines.

Hope that helps!

Update: I finally found the link to the shopping cart I had looked at. It's www.perlshop.org