OK, I'm going to assume that I can modify your Category class. If not, you can simply derive a new class from it, and place my additions in that.

First, lets stop using that nasty "field" method. We can simply define:

  sub name { shift->field('name') }
  sub sub_cat_ids { @{shift->field('sub_cats') }
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That'll get rid of a bit of the line noise.

Next, I want a more powerful 'new' operator. To avoid confusion, I'll name it new_tree. Later, we may rename it back to "new":

  sub new_tree
  {
    my $class = shift;
    my $id = shift;
    my $depth = shift;
    my $self = $class->new($id);
    $self->{_depth} = $depth;
    $self->{_sub_cats} = [
       sort { $a->name cmp $b->name }
       map { $class->new_tree($_, $depth+1) }
       $self->sub_cat_ids
    ];
    return $self;
  }

  sub sub_cats { @{shift->{_sub_cats} }
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With these methods in place, we can add one final method to return the flattened list:

  sub as_hash
  {
    my $self = shift;
    my $name = $self->name;
    my $depth = $self->depth;
    my $id = $self->id;
    return { depth->$depth, id=>$id, name=>$name };
  }

  sub as_flattened_list
  {
    my $self=shift;
    return
      $self->as_hash,
      map { $_->as_flattened_list } $self->sub_cats
  }
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Finally, we need to modify your BuildCatList function to use the upgraded Category class:

  sub BuildCatList
  {
    my $top = Category->new_tree("top", 0);
    my @cats_list = $top->as_flattened_list;
    my $cats = &main::GetDB('cats');
    $cats->{'~cat_list'} = &main::nfreeze( \@cat_list );
  }
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I've omitted all your undef checks; you can add them back if you want. --Dave.