'Bookending' things with square brackets creates an array reference (see perldoc perlref). So in the outer map, for each bookended thing, $_ is set to that array reference. @$_ dereferences that array reference, so my $item = pop @$_ removes the last item from the array (which is the subroutine reference), and puts it in $item. Then in the inner map we loop over everything left in the array (all the multiple keys) and create key/value hash pairs out of our multiple keys where the value is the subroutine reference that we popped off in the first place.