In "Asymmetrical Warfare" by S. R. Algernon, Earth is attacked by starfish-shaped aliens, who then wonder why the Earth warriors they killed aren’t regenerating. It’s an okay premise, but the problem is that all this story has is the premise. There are no characters except the narrator, whose only job is to wonder why the Earth warriors etc. It’s structured like a puzzle, but since we know why humans don’t regenerate there’s no tension, no point to trying to solve the riddle before the narrator gets there.
And how is it possible the aliens don’t know that some lifeforms don’t regenerate? Does everything in the universe except life on earth renew itself like this? If you can “split the body cleanly along the midline, to give it two chances at renewal,” as the narrator does, then can the aliens create two of themselves, and what do they do about overpopulation? But there really isn’t any point in asking these questions — this is a story about how starfish regenerate, and that’s pretty much it. I’d recommend it for a kid who’s interested in biology, but as far as the Hugo award goes, not so much.