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cat1
    cat1,sub1
...
cat2
cat3
    cat3,sub1

------------------------------------
| id | name               | parent |
...
| 6  | cat3               | 0      |
| 7  | cat3,sub1          | 6      |
------------------------------------

my @x =
('cat1',
...
 'cat3',
  ['cat3,sub1'],
);

$x[0] is SCALAR    1/0 (id/parent)
$x[1] is ARRAY
...
$x[4][0] is SCALAR    7/6
$x[4][1] isn't defined
$x[5] isn't defined