Many sound points.

With CGI.pm you can upload a file quite easily. Over a LAN it will be reasonably quick. As people have pointed out exporting Excel to another format and zipping the file will make the post go even faster.

Once you have the file on the server you can decompress it and stick it into mySQL. Using the suggested modules.

Given that an Excel file may take a while to upload, extract and store in mySQL, I'd also suggest copious logging, so you can track down what's going on if people complain. As monks have mentioned you need to give positive feedback to the broser asap, to prevent multiple submits and user frustration.

To avoid multi-click behaviour of users, merlyn has a recent WebTechniques article about one click processing, which is well worth reading, even if you don't use it in this project.