For example, what is '$cb'

The first argument. It's expected to be a code reference.

What I am trying to do is output the tree in a flattened format once the tree is built.

Based on the code from Re: Tree path analyzer regex problem (maybe other issues)?, a reply in an older thread of yours,

sub leaves {
    my $node = shift;
    my @children = $node->children();

    if (!@children) {
        print(join('|', @_), "\n");
    } else {
        for my $child (@children) {
            leaves($child, @_, $child->value());
        }
    }
}
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