How to disable temporary callbacks using Glade?

sub on_CoEn_changed{
        print "..Combo-Entry changed by user and NOT during set-Operations";
}

sub setSelector{
        signal_handler_block(?);                                                
#how?
        $form->{'Co'}   ->set_popdown_strings(@choice); #popDownList set        
    $form->{'CoEn'}     ->set_text($sv[2]);                         #e.g. value 
$sv[2] visible
                                                    #bad:calls on_CoEn_changed
    signal_handler_unblock(?);                      #how?
}

May be today is not my day, but i can't find the solution..

Any idea?

Thanks
wolfgang