use v5.14;

package Mouth {
	use Moo;
	has teeth => (is => 'ro');
	sub DESTROY { say 'Mouth->DESTROY' }
}

package ZombieHead {
	use Moo;
	has mouth => (is => 'ro', weak_ref => 1);
	sub DESTROY { say 'Head->DESTROY' }
}

my $mouth = Mouth->new(teeth => []);
my $head  = ZombieHead->new(mouth => $mouth);

say 'undefining $mouth';
undef $mouth;  # DESTROY is called!!

say 'undefining $head';
undef $head;