I've written many spreadsheets far larger than 242 rows with this module, so unless you are hitting the 7meg point, I'd say the problem is something else.

Your content type looks funny to me, I use

  print "Content-type: application/vnd.ms-excel\n";
  print "Content-Disposition: attachment; filename=${fn}\n\n";

  my $workbook  = Spreadsheet::WriteExcel->new("-");
[download]

As to a workaround, the technique I've used for spreadsheets that involve long db queries to build is to fork off the process in the background and email the resulting spreadsheet file to the user.

Scott