my %options = (
  main      => ["option1", "option2", "option3"],
  option1 => ["option1a", "option1b", "option1c"],
  option2 => ["option2a", "option2b", "option2c"],
  option3 => ["option3a", "option3b", "option3c"]
);

if ( !param() ) {
   $values_ref = $options{'main'};
} else {
   $values_ref = $options{param('distros_menu')};
}

print start_form();
print popup_menu(
        -name    => 'distros_menu',
        -values  => $values_ref,
);

print submit('which_distro_button');
print endform;