mod_perl keeps old values around between page invocations because it runs everything in one (or more) Apache processes. You can easily see if the problem becomes more treatable by launching Apache in single process mode:

httpd -X
[download]

Then, you should find that all visitors share the same shopping cart instead of wildly changing carts.

The most likely problem is failing to initialize global variables when you begin processing a page.

I recommend to move to all-lexical variables.