This construction (also used more times below) appears to add another block level (and thus complicate code optimization) in potentially hot code unnecessarily. Perhaps instead simply:

$synchronous ?? code(self) !! self.WHAT.start({ code(self) }, :$!scheduler)

or (just formatted a bit differently):

$synchronous ?? code(self)
             !! self.WHAT.start({ code(self) }, :$!scheduler)