Excel DWIMs, but the XLS file format does not. True to the format, Spreadsheet::WriteExcel will let you recreate the same cell format over and over, which may become a problem as there is a finite limit on the number of formats a given XLS file will allow.
Various modules that wrap around Spreadsheet::WriteExcel may or may not do the right thing. For example, it took 6 months and three programmers to do it right in Excel::Template.