In my experience I have not found a module that allows you to open a workbook, add or modify data, and write out to the same file.

If you're on a Windows system that has Excel installed, the Win32::OLE module will allow you to control Excel, which in turn means that you can open an Excel file, make modifications and save the modifications.

However, to do those tasks without directly using Excel, I would agree with you about not being aware of a single module to allow for read and write functionality.