At first glance this is a crazy question -- just test it out. If you look again it is really a question that asks -- how do I verify that my use of an external dependency will continue to work across versions? Now, that is not a silly question at all.

'Just test it out' is correct, but the how is the important part. If you have a critical script then it should be worth your time to modularize most of its operation and write tests. Add tests that specifically target the parts of those external modules that your script is relying on. For instance, Text::CSV has a fields() method that according to the pod will return an undefined value under certain circumstances. Then, when you want to validate that a newly released module will work, you can run it in an environment that uses the new version of the module and execute those tests. Learn to use Test::More, etc.