Method dispatch overhead. When you say foo( $a ) whatever foo in the current scope is called dirrectly. When you say $obj->bar( $b ), then perl has to check what $obj is (a scalar holding a blessed ref? a class name), then try and call Whatever::bar, and if that doesn't work try SuperWhatever::bar . . .
Granted there's some caching done, but it's more than just a simple subroutine call.