use File::Temp 'tempfile';
use CGI 'header';

my ($fh,$filename) = tempfile();
my $workbook = Spreadsheet::WriteExcel->new($filename);
do_stuff($workbook);
$fh->seek(0,0);
my $data;
my $size = -z $filename;
$size== $fh->read($data,$size) or bail_the_heck_out(); #CGI::Carp might help
print header(
    -type=>"application/vnd.ms-excel",
    -content_length=>$size,
    -attachment=>$file, # assuming you set that elsewhere
), $data;