note tilly As [koolade] [id://73933|pointed out], not all of these work. Yours in particular goes into deep recursion. Works in theory is not good enough, in golf you are going to learn a lot about the assumptions you cannot make. And yours has at least 2 separate (and serious) mistakes.<P> BTW the counts as I list them are just for the body of the sub, so if yours did work it would be 51 characters.<P> Playing independently I came in with several solutions that are small, and the following which is similar to yours really does work at 58: <code> sub f { my($s,$t)=@_;$t?$$t{d}eq$s?$t:f($s,$$t{l})||f($s,$$t{r}):0 } </code> And here, thanks to [koolade], is the test that I use: <code> $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, }, } }; sub test { my $val = f(@_); print $val ? "$val->{d}:$val\n" : "$val\n"; } test('e',$t); test('O',$t); </code> 73900 73921