sub circ_ref {
    my $dad = {};
    my $son = {};
    $dad->{son} = $son;
    $son->{dad} = $dad;

    Scalar::Util::weaken $son->{dad};

    return $dad;
}

my $dad = circ_ref();        # All is well
my $son = circ_ref()->{son}; # Doh! Who's your daddy?

print Data::Dumper::Dumper $dad, $son;