Ah. I was just copying an example from somewhere.

Really my experience with this kind of problem is that you keep trying variations that have worked for someone else and hopefully one of them finally does the trick.

Glancing at some code I have lying around the following definitely works with many people's copies of IE:

#! /usr/bin/perl -w
use strict;
use CGI;
print CGI->header(
    -type => "application/vnd.ms-excel",
    -attachment => "example.xls",
);
print "Salutation\tRecipient\nHello\tWorld\nGreetings\tEarthlings\n";
[download]

So if that doesn't work I can guarantee that the problem is specific to his copy of IE, and is not shared by IE in general.