An Excel file is a binary file within a binary file. It contains several interlinked checksums and changing even one byte can cause it to become corrupted.
As such you cannot simply append or update an Excel file. The only way to achieve this is to read the entire file into memory, make the required changes or additions and then write the file out again.
You can read and rewrite an Excel file using the Spreadsheet::ParseExcel::SaveParser module which is a wrapper around Spreadsheet::ParseExcel and Spreadsheet::WriteExcel. It is part of the Spreadsheet::ParseExcel package: search.cpan.org/search?dist=Spreadsheet-ParseExcel.
However, you can only rewrite the features that Spreadsheet::WriteExcel supports so macros, graphs and some other features in the original Excel file will be lost. Also, formulas aren't rewritten, only the result of a formula is written.