It sounds like no matter what, you're going to be relying on the remote user. To make things most simple, you could supply the remote user with a perl script that will extract the data needed, and then encrypt it to send to you. You'd be in control of the encryption algorithm used at both ends this way.

Another option is to have excel spew its data into a cvs, then encrypt that file using whatever method you wanted so long as it was reversable.

And finally, a bit of VBA like Taulmarill wrote above could be used to export the file in an encrypted way, but this is probably the most complicated and slowest option to code. (meaning getting the code working will take the longest, I'm not attempting to make predictions of which option is going to be the fastest once written).

-Scott

Update: The Email encryption eXile has suggested above is also a good option I hadn't thought of. It reminded me of yet another option: Is the remote user using VPN to connect back to you? If so, the network is already encrypted, and you wouldn't need to encrypt that file before transmitting.