It's usually left out of the examples/description/synopsis in modules on CPAN too, probably for the sake of simplicity and clarity. I notice that a lot of the posts here leave it out (probably for the same reason), even by people who I'm sure use them when they write real code. It would say it depends on the nature of the code; is it demonstration, or is it meant to be used as-is?

So no, I don't immediately discount code if it's missing a use warnings.