That's called a pre-order traversal, because you deal with the key before dealing with the children. I typically do something like this:

    $tree = [[], $key, []];

    sub traverse;
    sub traverse {
        return unless @{$_ [0]};
        traverse $_ [0] [0];
        print    $_ [0] [1];
        traverse $_ [0] [2];
    }
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That's an in-order traversal. Your structure has the advantage that it can deal with any number of children, while the code above assumes 2 children for each non-empty node; as is common for binary trees.

Abigail