What makes that perfectly round hole in the opened clamshells that I find on the beach in Maine?

Most likely it's from the radula of a Moon Snail. The snails hunt and eat bivales by grabbing the clams with their foot and then boring a hole through the shell with their Radulated tongue.

Hopefully you can still rake a few clams for your chowder.

Cheers

A drill so it can be on a Necklace. Or
2 days ago I found a Clam Shell on the Beach that had a hole, not natural(normal), I kept it. I have no reason to doubt Sitkadiver. I also keep rocks have holes through them?

Snail shells on California beaches have the same holes. I just picked up a half dozen yesterday.