Sometimes xlsx files are misnamed as xls files. You can tell by opening the file in a text editor. If the first two characters are 'PK' (meaning it's a PK zip file which is what xlsx files are), then it's an xlsx file. I use Spreadsheet::ParseExcel::Stream partly because it will correctly open misnamed xls(x) files. It still won't open if the file is really a csv or some other format.