I've been using HTML::FillInForm for ages now, but only in conjunction with HTML::Template and the cool plug-ing that comes with CGI::Application. Anyway, pulling from the docs:

my $output = HTML::FillInForm->fill( \$html, \%fifvalues);
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If look at the above, note 3 main concepts:

1. You need your HTML in a variable* (e.g., $html built by hand, CGI, or HTML::Template in your Perl script).
2. You need the form value(s) in a variable (I always use a hash ref because I'm always pulling from a database, e.g., $fifvalues->{'gender' => 'male'}) assuming your radio button is named "gender" and one of the values is "male".
3. Then you will need to print the $output, e.g., print "Content-type: text/html\n\n"; print $output;

So, forget checked, it will be done for you. But just try it, that's how I learned.

*Update: You can just reference an actual static html page with a form, negating the need to build it with the script.