open PASSWD, "/etc/passwd" or die;
  @passwd = <PASSWD>;
  @key = map                            # Create an array @key containing
    { (split /:/)[2] } @passwd;         # just the keys (user ids).


  @by_uid_index = sort {                # Sort indices, using @key array.
    $key[$a] <=> $key[$b]
  } 0 .. $#key;

  @by_uid = @passwd[@by_uid_index];     # Now, rearrange the contents of
                                        # @passwd into sorted order.

Or just combine the last two statements:

  @by_uid = @passwd[sort { $key[$a] <=> $key[$b] } 0 .. $#key];