But it seems bizarre to me that it "can't" do this because some fairly rare programs dynamically write and name a subroutine. That can't happen very often.

The counter example in the thread you linked is not that. It is that a module which exports or even just provides a given sub may only be dynamically loaded at runtime. This happens more often than you appear to presume.

If your test suite doesn't have 100% coverage then this is the sort of thing that will bite you occasionally. The only guaranteed solution is ... improve the coverage of your test suite. Not the advice you wanted to hear, no doubt.