$t = {
    d => 'd',
    l => {
        d => 'b',
        l => { d => 'a', l => 0, r => 0, },
        r => { d => 'c', l => 0, r => 0, },
    },
    r => {
        d => 'f',
        l => { d => 'e', l => 0, r => 0, },
        r => { d => 'g', l => 0, r => 0, },
    }
};

##</code><code>##

sub f{r(pop);$a{(pop)}||0;sub r{my$n=shift||return;$a{$n->{d}}=$n;r($n->{l});r($n->{r});};};