If you'd previewed your post, you would have seen that the code segment didn't come through correctly. I'm guessing it should have look like this:

my $attachment_name = $start_hour . "_" . $hour_count;
print $cgi->header(-type=>"application/vnd.ms-excel",
  -attachment=>"$attachment_name.xls");
print $cgi->start_html(
  -title=>"Exported Excel results"); # Write the data
print $cgi->end_html;
[download]

It's a little difficult to figure out what the problem is without more information, but it sounds like this solution used to work, but is now failing. Apart from the missing file, do you have any error logs to tell us about? Does the code still compile? Are you using something other than Excel::Writer::XLSX to create the file? Are you using strict and warnings?