if ( !param() ) {

    print start_form();
    print popup_menu(
        -name    => 'distros_menu',
        -values  => \@distros,
        -default => 'centos'
    );
    print submit('which_distro_button');

    print endform;
}
else {
    $which_radio_button = param('distros_menu');
    print $which_radio_button;
    $q->end_html;
}