use Win32::OLE;

# If Excel is already running it uses the copy in memory
eval {$ex = Win32::OLE->GetActiveObject('Excel.Application')};
if ($@) {
   $ex = Win32::OLE->new('Excel.Application', sub {$_[0]->Quit;})
   	   or die;
}

# open an existing workbook
$book = $ex->Workbooks->Open( 'yourdoc.xls' );

#write to a cell
$book->Worksheets(1)->Cells(1,1)->Value = "whatever";

#save and exit
$book->Save;
undef $book;
undef $ex;