Use something like LiveHTTPHeaders (or wget) to see if your Content-Disposition is being set correctly -- I haven't tested it but I think you may need to do Content_Disposition. Or just use the -attachment parameter:

 print $cgi->header(
  -type => 'application/vnd.ms-excel',
  -attachment => $file,
  -Content_Length => $filesize
);
[download]

update: Either using -Content_Disposition or -attachment works for me.