That was just a simplified example. The point is that if you have a $ proto, arguments are evaluated in scalar context.
What if someone's invoking a sub with localtime as an argument, for instance?
I agree, it's possible that's not happening. But without a heck of a good test suite, I'd hesitate to make that kind of change on a project of any size. If there's a wantarray buried in a sub somewhere, it's going to make for a tough problem to debug...