not really perl-solutions, but after some thought -
well... changing the mime-types won't work, since some browsers disregard this in favour of their own results from their pre-scanning of the file - and changing the extension of the file would probably confuse visitors... or scare'em.
setting up some ftp-thingee might be a thought - but really ->
the easiest thing you can do is to compress it (if you don't wanna use tar.gz - use WinZip - or better yet, have both options available). Zipped files are always (i would think) downloaded like ya want.
OR... simply put a link to the document and tell your users, that if they'd like a copy - they should right-click on the link and select SAVE TARGET AS (or whatever). That'd be the easiest.