#!/usr/local/bin/perl

use strict;
use warnings;

my %category_hash;
my %categories;

# Build a hash containing all of our data records...
while (<DATA>) {
  chomp;
  my %cat;
  @cat{'id','name','parent'} = split /,/;
  $categories{$cat{id}} = \%cat;
}

# ... and turn it into a tree
foreach $_ (keys %categories) {
  if ($categories{$_}{parent}) {
    $categories{$categories{$_}{parent}}{children}{$_} =
      $categories{$_};
  } else {
    $category_hash{$_} = $categories{$_};
  }
}

# And now %category_hash is exactly the same as it was
# in my previous post. Carry on with "show" as before.

__DATA__
1,whatever,0
2,something,1
3,another subcategory,2
4,something else,0