Hard to mistake a walleye for a YP, even for an unexperienced fish squeezer... I guess we'll never know!
My personal opinion is if it was weighed, measured, and witnessed by a C.O. it's most likely legit, and I don't think a C.O. could have mistaken a walleye for a yellow perch.
I also believe that a 21 inch perch could weigh only 3 lbs. 12 oz. Any perch that reached 21 inches would be a mighty old fish, and I think we've all see old skinny fish. I took in a 25 inch largemouth once to mount that was all head and weighed only 3 lbs.