Neither _utf8_on (in Encode.pm) nor utf8_on (in DSU) mention a return value, _utf8_on (in DSU, probably different from Encode's) explicitly says

The data structure is converted in-place and as a convenience the passed variable is returned from the function.

Given mpersico's observation, it looks like for the hash itself it is not relevant whether its keys have an utf8 flag set or not.