Never expect a developer to change the file's settings on checkin/checkout reliably. "Oh, what I'm doing is just such a quick change, I'll skip that."

You might think that checkin/checkout hooks are the answer. Under that scheme, you make the version control system tweak the code on checkin/checkout for you. Example: checking out to a developer box means "use strict;" checking out to test servers means "use strict;" checking out to production servers means "no strict." Until you forget to add or remove certain usernames from a list. And don't forget the heisenbugs which somehow only show up when there's no debugger around.

No, version control is not a part of the code. Version control is insurance against mistakes and disasters, but not a tool which actively tunes your applications. Just because it's possible doesn't mean it's a good practice.