sub tree_data {
  my ( $old_data, $key_name, $next_group, @remaining_keys ) = @_;
  
  # guard against bad input and serve as recursion end case
  return $old_data unless defined $key_name;
  
  my @new_data;
  my $last_key = '';  # if this is initialized, you dont have to check below
  my $build_box = []; # initalize this explicity (more self documenting)
  
  # encapsulate code into inner sub
  # fortunately for us, all variables 
  # are closed over too :)
  my $push_new_data = sub {
      return unless $last_key;
      push @new_data, {
          $key_name   => $last_key,
          $next_group => tree_data( $build_box, @remaining_keys )
      };
      # re-init this 
      $build_box = [];
  };

  foreach ( @{ $old_data } ) {
    if ( $last_key ne $_->{ $key_name } ) {
      $push_new_data->();
    }
    $last_key = delete $_->{ $key_name };
    push @{ $build_box }, $_;
  }
  $push_new_data->();

  return \@new_data;
}