In theory you have to do the following:
- Configure the server so when it receives an e-mail from specified addresses it should execute the Perl script passing the content of the e-mail as a param;
- Make the Perl script read this param, discard 'useless' information from it (like the mail headers) and append the rest to a file. If you're talking about an Excel spreadsheet, you'll probably consider using Win32::OLE for the append step.
I'll have to do something similar probably a month from now, so if you still need assistance then we can work together, let me know.
If you are still looking for something that was already made, keep googling. I'm sure that there's a lot of stuff out there.