Am I right in thinking that the only way to test each individual sub would be to create a module eventhough if the code is not re-usable by any other scripts (as suggested by Thilosophy)?

All you have to do to test individual subs is to load a pile of code somehow and control the flow of execution from something other than the program itself.

Granted, this is easier if you create one or more modules, but it's not a requirement. (The purpose of modules is to help you organize code, not just to reuse it.)