use Win32::OLE;
use Win32::OLE::Const;
#...
my $ex_ol = Win32::OLE::Const->Load("Microsoft Excel 10.0 Object Library");
my $xcl = Win32::OLE->new('Excel.Application','Quit');
#later
$xcl->Application->{DisplayAlerts}  = 0;
$doc->SaveAs($outputFileName, $$ex_ol{'xlWorkbookNormal'});