I think I was clear that in the general case, as long as the subroutine is not invoked as part of the 'use', there is no problem. If the subroutine is invoked, then it needs to have already been defined. Only taking the reference of an as-yet-non-existent subroutine is not dangerous.

Actually, there is one danger, and it has to do with prototypes. If the prototypes differ (when the function actually is defined), Perl will croak.