I think you may have a confused definition of what it means to be a rational actor. Being rational means making the optimal decision given the information available

No, you're confused. See

https://en.wikipedia.org/wiki/Bounded_rationality

Maybe I don't understand what you mean by "perfectly rational". I'm using the definition from the article: A perfectly calibrated individual will be right X% of the time about statements in which they are X% confident.

Are you using a different definition?

Perfectly rational means just what it sounds like. Making the correct decision because you have all available data.

Being perfectly rational is impossible.

Would a perfectly rational person duck if there were a 50% chance they were about to be punched? Or would they do nothing?

A perfectly rational person would have all available information. Since ducking wastes energy vs. doing nothing, if they were going to get hit they would duck, and if not, they would do nothing.

But again, being perfectly rational is impossible.

I don't know how to make it any clearer.