Or worse still, you're writing a child class and carefully check that the parent class doesn't define _frobnicate; everything seems good. Then you install an upgrade of the parent class from CPAN, and it defines _frobnicate.

If the updated parent class had made frobnicate a public method then the problem would be pretty easy to track down, maybe even by just reading the Changes file. If the updated parent class had used my $_frobnicate = sub { ... }; this wouldn't be a problem to begin with.