Maybe this is helpful?

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;

[download]

Compare that with the ZombieHead which allows its mouth to randomly disintegrate by using a weak reference...

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;

[download]

References can be weakened using Scalar::Util.

package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name

In reply to Re: when is destroy function called by tobyink
in thread when is destroy function called by david2008

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