Form Validation

[John Pretti]

Hi all,
 
What is the best way to validate a form element? I am using the following
code to generate a set of radio buttons; how can I ensure that one of the
buttons is checked before processing the rest of the script?
 
print   $q->start_multipart_form,
                $q->p("Please select a directory to promote the file:"),
                $q->radio_group (
                        -name=>'promdir',
                        -values=>
['hw','nw','sw','swmfis','swunifi','swwebcaaf'],
                        -labels=>\%label,
                        -default=>'selected',
                        -linebreak=>'true'),
                $q->br,
                $q->submit('Promote'),
                $q->end_form;
 
Thanks in advance,
John