XLSX files are just zipped XML files (plus some other components). You could try setting up a simple worksheet with your desired formatting, and then extract the file contents (I use 7zip on Windows) and view them using a text editor to see what is expected.

Look under the xl/worksheets subdir to find the actual data.

Edit - although that is for setting the rules directly in the file. Maybe there is a more effective approach using Spreadsheet::Excel.