use Encode;

sub cgiapp_postrun {
  my($this, $response_ref)=@_;

  my $encoding = ... #figure out which encoding to use

  if($encoding ne "UTF-8") {
       my $response_utf8 = decode('UTF-8', $$response_ref);
       $$response_ref = encode('ISO-8859-1', $response_utf8);
  }