I see that you're fetching categories in groups, grabbing all the children of a specific category, then going on to the next parent category... My inclination would be to fetch all the categories at once, then organize them. Here's one way to do this: 
my $cats_sth = $dbh->prepare(<<"EndOfSQL");
  SELECT category_id, name, parent
  FROM   $sql{categories}
  ORDER BY category_id ASC
  -- assumes parent id is always less than child id
EndOfSQL

my @cats;

# build the data structure
while (my($cat_id, $name, $parent_id) = $cats_sth->fetchrow_array()) {
    $cats[$cat_id] = [$name, $parent_id, []];
    if ($cats[$parent_id]) {
        push @{$cats[$parent_id][2]}, $cat_id;
    } elsif ($parent_id) {
        warn "Parent $parent_id not found for category $cat_id '$name'
+\n";
    }
}

# print the categories
foreach my $cat (@cats) {
    next unless $cat and !$cat->[1];
    print_category($cat, \@cats, '');
}

# recursively print a category and its children
sub print_category {
    my($cat, $cats, $indent) = @_;
    
    print $indent, $cat->[0], "\n";
    foreach my $child_id (@{$cat->[2]}) {
        print_category($cats->[$child_id], $cats, "$indent  ");
    }
}

[download]
The data structure that this uses is: 
[
   [ name, parent id, [ child ids ] ],
   [ name, parent id, [ child ids ] ],
   ...
]

[download]
where the ids are also the indexes into the array.

This probably is not the best data structure for your task, although it does get the job done. It should give you some ideas about solving the problem.

In reply to Re: Collecting data in a recursive routine. by chipmunk
in thread Collecting data in a recursive routine. by Anonymous Monk

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