Better hotlink protection

I upgraded the hotlink prevention on tetrap.com to use a method suggested by Smarter Image Hotlinking Prevention on A List Apart. I have a lot of trouble with hotlinkers, mainly from message boards and people linking to album covers from EBay.

The new method allows linking to pictures like so but prevents the use of the img tag for imbedding. I've implemented the script in Perl rather than PHP, and it also checks to make sure the image exists (and returns a 404 if it doesn't), so is a slight improvement over the A List Apart method.