merrymonk, what version of Excel are you using and which version of Excel file are you wanting to save it as? Based on the error message that you described, it sounds like you saved the Excel file in one version format and when you manually opened the file, Excel was expected a different version format.

• If you're using Excel 2003:
  • You can only save the file as Excel 2003 format, which means you need to use .xls file extension.
• If you're using Excel 2007 and want to save it in Excel 2003 format:
  • You need to use the .xls file extension and need to add FileFormat parameter to the SaveAs method.
  • Example: $workbook->SaveAs(Filename => "C:\\test.xls", Fileformat => xlExcel8);
• If you're using Excel 2007 and want to save in Excel 2007 format:
  • You need to use the .xlsx file extension.
  • I forgot the full reasons behind it and it might not apply to your situation, but one of my scripts has an additional parameter for the SaveAs method in this scenario.
    Example: $workbook->SaveAs(Filename => "C:\\test.xls", ConflictResolution => xlLocalSessionChanges);

Hopefully this information will help point you in the right direction in debugging the problem.