sub flatten_recursive {
  my $tree = shift;
  my @result;
  for my $node (@$tree) {
    push @result, ref $node ? flatten_recursive($node) : $node;
  }
  @result;
}

print join ' ', flatten_recursive($data_struct), "\n";

sub flatten_iterative {
    my @tree = ($_[0]);
    my @results;
    for my $node (@tree) {
        (ref $node) ? push @tree, @$node : 
                      push @results, $node;
    }
  return \@results;
}

print join ' ', @{flatten_iterative($data_struct)}, "\n";