sub cgiapp_prerun {
 my($this, $rm)=@_;

 eval {
    ...
 };
 if($@) {
   $this->stash->{exception} = $@;
   $this->prerun_mode("prerun_error")
 }
}

sub prerun_error {
 my($this)=@_;

 $this->error($this->stash->{exception});
}

sub error {     # error-runmode
 my($this, $exception)=@_;
 ... 
}