That's a good guess. If you place a "warn" inside of BEGIN, UNITCHECK, CHECK, INIT, and END blocks, you'll see that the "only once" warning comes immediately after the BEGIN, but before UNITCHECK. It's being generated before runtime, so by the time the subroutine is actually called, it's too late.
Those solutions that deal with import are probably the best answer, though it wouldn't fix fully-qualified usages of reduce (where there is no import call).
I think a better solution would simply be for Perl to completely ignore $a and $b with respect to "used only once" warnings. Those should be additionally special-cased. Now to code-dive again and find where that could be implemented.