I've never used IIS, so I can't be a lot of help with that. Having said that, I suspect (from "only saw 200 there even when the page broke") that particular 200 refers to successfully rendering the page which contained little more than '<p>Page cannot be displayed</p>': lines in the log just prior to that may be more informative.

The message "Page cannot be displayed" is intended for general users, not developers. Use whatever development tools are provided by your browser to get a more informative message.

I'd recommend adding 'use warnings;' to all your scripts (unless you have a very good reason not to). While you're experiencing difficulties in the development phase, you might want to make that 'use warnings FATAL => 'all';'.

Try cutting your CGI script down to the bare bones (i.e. no spreadsheet processing or file uploads) and ensuring that works. Then try uploading a single, existing file from a known location and sort out any bugs there. Once all that's working, add the code to generate the upload file and continue testing. Doing this, in a stepwise progression from simple to complex, will mean basic problems will be found early on and you'll be testing new code at each stage rather than trying to test everything at once.

Have you looked in the CGI documentation? Many of its sections look like they may be useful in this specific instance, including (but not limited to): DEBUGGING, CREATING A FILE UPLOAD FIELD and PROCESSING A FILE UPLOAD FIELD.

The following threads (on this site) are a little old but may still provide useful information: Troubleshooting Perl CGI scripts and CGI Help Guide.