# Einfacher Hash
my $foo; $foo = {foo=>\$foo};

# Komplizierterer Hash
my $bar = {
    blubb => {}
};
$bar->{blubb}{ding}{dang} = \$bar->{blubb};

# Closure (besonders trickreich)
my ($sub1, $sub2);
$sub1 = sub {sub2->();};
$sub2 = sub {sub1->();};