use v5.14;

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

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

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

say 'undefining $mouth';
# Note: DESTROY not called, because $head still refers to $mouth
undef $mouth;

say 'undefining $head';
# Now both DESTROY methods will be called
undef $head;

##</code><code>##

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;