Your "closure" sub isn't actually a closure; to be a closure it would not only have to access outside lexicals (like yours does), but also be anonymous
. What you have works anyway because you don't attempt to have several independent swap subs each of which has its own private state.
BTW, the reason why this doesn't work:
$result = $head. " er glad " . $result while( my $head = &swap );
... is that the parsing works left-to-right, and so $head has to be declared by the point where it is first seen in scanning, not where it would first be touched in execution.
-- Chip Salzenberg, Free-Floating Agent of Chaos