if ($data[$i]=~ m/^\.\.BD/) {
    unless ... {
      $collect .= ',' . $data[i+1];
    }
  }
  else {
    print $collect,"\n" if ($collect);
    $collect='';
    if ($data[$i]=~ m/^\.\.AU/) {
      unless ...
        $collect= $data[i+1];
      }
    }
  }
}