sub visit_preorder {
   my ($cb, node, $depth) = @_;
   $depth ||= 0;
   $cb->($node, $depth);
   visit_preorder($cb, $_, $depth+1) for $node->children();
}
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For example, what is '$cb' ? What I am trying to do is output the tree in a flattened format once the tree is built. Something like : Fruit|Apple|Granny Smith.. Fruit being the parent of Apple, Apple being the parent of Granny Smith..