"The more headweight is pushed out farther from the center of gravity of the bat, the higher the bat's MOI about the vertical rotation axis of its center of gravity," Wishon said.
"And the higher the bat's MOI, the less the bat will twist in response to an off-center hit. And the less bat twisting from an off-center hit, the farther the ball will fly for the same swing speed. So perimeter weighting 'works' by increasing a bat's MOI, thereby leading to less loss of distance on off-center hits."
Found this description of perimeter weighting in gold clubs, have enerv actually managed to find any evidence online though. Not saying it doesn't work in theory, but I like cold hard proof!