I ran in to a similar ( I think :] ) problem on a script I was doing for an HTML page that would allow the user to update items for sale, and delete them when they were sold.

Since I had checkboxes, I needed a way to make them unique. I ended up doing generation on the fly manually. There was no way for CGI.pm to do it as I was basically saying 'd'.$index, and I was incrementing index for every item in the base file.

So in answer to your question, if you need it to be on the fly, you're probably going to have to do it by hand. It makes your not quite as clean as CGI.pm is going to make it look, but them's the breaks :)

Hope that helps you out.