if they are not overridable then it returns undef - but then please
note that the inverse implication does not hold.
yes, those with an undefined prototype are still overridable, but
you can't make the replacement function behave like the original one
in every way (due to parsing issues), for example exec/system's indirect object syntax.
BTW, I was once also in desperate need to override
the filetest operators... so I'm pretty sure it cannot be achieved with
the current implementation — but also see the patch mentioned in
footnote 2 in the post I linked to.