General :: Write A Script That Will Change Just One Item?

Im trying to post some data with curl. What I have is an html form that has many items. When I use a web browser to post data I can change one item easily. When I use curl (or wget) it will change the item that I tell it to but all other items in the form are erased. Im trying to write a script that will change just one item and preserve everything else. The problem is that I dont know what the values are before hand. Every example ive seen uses firefox addons or some other method to tell what to post. This would be fine but is not scriptable.