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sub traverse_tree {
    my ( $tree_root, $visitor_callback ) = @_;
...
        ( $prev_node, $curr_node ) = ( $curr_node, $next_node );
    }
}

sub traverse_tree {
    my ( $tree_root, $visitor_callback ) = @_;
...
        ( $prev_node, $curr_node ) = ( $curr_node, $next_node );
    }
}