As suggested already, you will get lots of hits using Super Search. Also be sure to check out the couple links that are in the Tutorials, and also the node Index of Spreadsheet FAQs contains lots of good information and references.

As for your specific issue, note that the Spreadsheet::WriteExcel pod states in the description that "This module cannot be used to write to an existing Excel file." So I also recommend the Win32::OLE approach (including porting macros), or I would create Spreadsheet::WriteExcel code that can recreate your workbooks from scratch as reusable code, and just call that each time to prep your new workbook, add the new worksheet, and write it out.

Also see the thread write over existing Excel file from a few days ago, including my reply to it.