You could get it from an object that overloads numification (0+) or int

Yes, that would do it.

I did find a case where Scalar::Util::looks_like_number and the OP's isNumber return a different value for the same argument, but it had nothing to do with emission of warnings.

For the arguments Math::MPFR->new(), Math::MPFR->new('nan') and Math::MPFR->new('inf'), Scalar::Util::looks_like_number will return FALSE, while isNumber will return TRUE.

But this is probably the fault of Math::MPFR, which stringifies all values the same way as the mpfr library.
And those 3 expressions therefore get stringified as "@NaN@", "@NaN@" and "@Inf@" (respectively).

Math::MPFR probably should fix this by doing a s/@//g on the string prior to returning it. (And I'll attend to that.)

I'll also mention that there's a perl API subroutine also named looks_like_number, but it doesn't look for any magic and therefore returns FALSE for most objects.
Actually, I don't know of any cases where it will return TRUE for an object ... but I'm wary of asserting that such a case doesn't exist, or couldn't be constructed ...

Cheers,
Rob