I guess it's one expression but it has a block and a semicolon in it so... (lol)

I think you are right. When I tried to do keys(%A, %Z) it failed its arg magic and it was as if I had called keys("a", 1, "b", 2, "c", 3, "z", 9, "y", 8, "x", 7). It looked at its "one argument" ("a") and threw the "no scalars" error. I wouldn't call it a "bug" but it's a pretty misleading/confusing error message.

I'm not worried about the sizes. In general, an excellent piece of advice, but in this case there will be literally twos of keys in each hash. I just wanted the union quick and easy.

To your last point, sure, I use hash overlays on purpose all the time. In this case I only care about the keys, though.