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my $data = [
   [ 'parent1', 'child1', 'grand_child1', undef,    ],
   [ 'parent1', 'child2', 'grand_child2', 'great_grand_child1',],
   [ 'parent1', 'child3', undef, undef,],
];

my $data = [
   [ 'parent1',      'child1' ],
   [ 'child1',       'grand_child1' ],
...
   [ 'grand_child2', 'great_grand_child2' ],
   [ 'parent1',      'child1c' ],
];