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my $data = [
   [ 'parent1', 'child1a', 'child2a', undef,    ],
   [ 'parent1', 'child1b', 'child2b', 'child3b',],
   [ 'parent1', 'child1c', undef, undef,],
];

my @results;
foreach my $row (@$data) {
   push @results, treeify(@$row);
...
   return {} unless $node;
   return {$node => treeify(@list)};
}

$VAR1 = {
          'parent1' => {
                         'child1a' => {
...
                         'child1c' => {}
                       }
        };