If attributes::import returns a value, funny things happen.

Also visible in Pure-perl:

$ perl -wE 'sub MODIFY_SCALAR_ATTRIBUTES {}; sub add { my ($x, $y) : Bar = @_; return $x + $y; } add(7, 8, 9, 10, 11)'
Useless assignment to a temporary at -e line 1.
Useless assignment to a temporary at -e line 1.