I think you can accomplish your goal by using a combination of two CPAN modules: Spreadsheet::ParseExcel and Spreadsheet::WriteExcel. Use ParseExcel to get the data from the 20 excel files while using WriteExcel to add the data (organized as you like) to a new sheet using WriteExcel.

Since the columns are not in the same order on each sheet you'll need to get the value of A1, B1, .... G1 for the names of the columns. As long as the naming convention is the same in each sheet you should be able to keep all the data together by an IF statement.