Since you're using CGI, you probably should make use of the methods that it provides for this sort of thing instead of using the environment variables. It's not going to make much of a difference under Apache, but if you ever try to port this to another Web server platform, you'll be glad you did this:

my $path = $cgi->path_info();

This should return any parameters received after the CGI program itself. Give it a bit of testing to see how to map this back in to your cart namespace.