$r->err_headers_out->{'Content-Disposition'} ="attachment; filename=\"$csv_file\"";
  $r->headers_out->{'Content-Disposition'} ="attachment; filename=\"$csv_file\"";
  $r->headers_out->{'ABZZ-Disposition'} ="attachment; filename=\"$csv_file\"";
  $r->content_type('text/csv');
  $r->set_content_length([stat $csv_fh]->[7]);
  $r->rflush();

And then send the file via this:
 $r->sendfile($pdf_file);