I don't know for sure, but the simple reason would be that no parameters are being passed to the script. You could test this, even just to rule it out, by putting a hidden field in your form with a known value or even putting some query params at the end of the 'action' line in your form tag. I wouldn't recommend doing this permanently, but it can help debug. For example, in your form, you could:

<form action="http://my.site.com/script.pl?test1=one">
[download]

How are you testing whether the params are coming back? Just printing back to the browser? Because it's also possible that you are getting them, but you aren't displaying them properly. Can you show some code?