The problem isn't Perl. The problem is IE, up to V5 (I don't know about 5.5 or 6 beta) does not support multi-part mime server responses. Depending on how you want your splash to work, you might try something like this on your splash page:

<meta HTTP-EQUIV="Refresh" Content="3;URL=...>
[download]

This causes the page to be displayed for 3 seconds and then the specified URL is loaded.

Have fun,
Carl Forde