my %by;
  @by{ map $_->{num}, @$data } = 0 .. $#$data;

  @sorted_unique = @{$data}[ @by{ sort keys %by } ];

##</code><code>##

  my %by = map{ $_->{num} => $_ } @$data;

  @sorted_unique = @by{sort keys %by};

##</code><code>##

  @sorted_unique =
    @{{ map{ $_->{num} => $_ } @$d }}{ sort map $_->{num}, @$d };