One basic principle to follow is to never trust anything from the browser that you can look up elsewhere with more confidence. Imagine a shopping cart application, in which a user browses through items for sale and adds them to his cart, finally checking out. If you store the price or shipping costs in the user's cart and trust those values blindly, you may find yourself selling items for a penny or even (if you're particulalry careless) paying customers to buy your stuff. Better to accept the minimum information from the user (perhaps an item-identifying number and a quantity) and re-calculate cost & shipping every time you need it.

This is a rather simplistic example, but I think that the principle is sound -- never trust tainted data and be very careful about how you detaint it. Some folks use CGI::Session to help with this sort of thing.