A browser may be set to download an excel file instead of launch a spreadsheet application. So you'd need to have fine grain control of your users browsers (and platform choice) in order to launch Excel.
Spreadsheet::WriteExcel may interest you.
You need to set the mime type. Here's a header method I use (in mod perl).

sub print_html_header {
        my $self = shift;
        if ($self->style eq "excel") {
                my $filename = $self->new_excel_filename;
                $self->{r}->header_out('Content-Disposition'=>"attachm
+ent; filename=$filename")
        }
        my $content_type = ($self->style eq 'excel') ? 'application/vn
+d.ms-excel'
                : 'text/html'; 
        $self->{r}->send_http_header($content_type);
}
[download]