But with strict that already doesn't compile. When you use strict, "once" usually becomes pretty irrelevant.

Mostly yes, but not if you refer to a fully qualified varable from another package. For example, when I am debugging my code, I often put DB::single=1 at various places. Sometime I leave those line in production code inside assert like error checks, as a kind of assert that will only go off when debugging.

I have found that those lines will trigger the "once" warning, which is irritating.