it isn't browser-specific. I've tried it in opera 10.01 and FF 3.5. When the page first loads , nothing is checked, but as soon as something is checked it is impossible to clear it ,using the CGI.pm constructor.
not sure of --override syntax. the variations ive tried haven't made any difference.
Honestly, what you are trying to do doesn't make much sense. A radio input is intended to select one of many. Not zero, not two. I'm not saying it's not possible, just that it wouldn't surprise me if it isn't.
Why don't you add an extra button name "None" or similar.