Your code concatenates several excel files into the output file. I'm surprised that Excel accepts this broken file as input. The Excel file format is a binary dump, simply appending yet another dump does not work. If you want to CHANGE the data inside an existing file, you need to read that file into memory, modifiy the in-memory-representation of the file, and write back the changed structure as a single binary dump to the file, replacing the original dump, just as Excel does. Spreadsheet::ReadExcel may help. Choosing a different file format is another option. Simple CSV files can be appended to, but they don't give you formulas, macros, and formats.

Alexander