Without specific code examples, I can only give a general answer but, basically, when you're generating the "previous" page, you need to detect (in your Perl code) whether or not the radio button should be disabled, and give the radio button the attribute of <input disabled ...>.

If you're using CGI for your HTML generation (I would recommend templates instead), you may need to set / clear the sticky option, as suggested by the previous answers, but probably not. (IIRC, it defaults to on, and that should work fine.)

Update: Alternatively, you could add to your javascript onLoad handler a call to the function that disables / enables the radio button.

bbfu
Black flowers blossom
Fearless on my breath