I dare say your join is broken and that's what should be fixed. But it's easy to work with it anyway.

You have: 

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

[download]

You want: 

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

[download]

Easy! Change 

    while (my $row = $sth->fetch()) {
        my ($id, $parent_id) = @$row;
        ...
    }

[download]

to 

    while (my $row = $sth->fetch()) {
        my ($parent_id, @children_ids) = @$row;
        for my $id (grep defined, @children_ids) {
            ...
        }
    }

[download]

In reply to Re^3: Generating a Hash of Hashes Recursively to Display Database Hierarchy by ikegami
in thread Generating a Hash of Hashes Recursively to Display Database Hierarchy by c4onastick

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