I'm curious why you think the worm's length needs to be smaller than the distance between a frog's eyes. It's the width that matters, and even then it hardly matters with something soft like a worm. Frogs are also quite capable of safely swallowing things larger than the distance between their eyes, and there's no danger of a cloacal prolapse caused by passing feces from large prey with something like a worm that pretty much gets digested into diarrhea- like goo. Worms cut into such tiny pieces probably won't move enough to attract the attention of a frog that hasn't been trained to eat whatever you put in front of it.