sub cgiapp_prerun {
  my $this = shift;
  my $rm   = shift;

  eval {
    ...
  };
  if($@) {
    $this->stash->{exception} = "Error in prerun: $@";
    $this->prerun_mode("error");
  }
}

sub error {     # error-runmode
  my $this = shift;
  my $exception = shift        || 
     $this->stash->{exception} ||
     "default error message goes here";

 ... 
}


##</code><code>##

$this->error("danger! red alert!");

##</code><code>##


# check 'name' entry
unless ($name) {
  $this->stash->{exception} .= "bad name\n";
}

# check 'address' entry
unless ($address) {
  $this->stash->{exception} .= "bad address\n";
}

# check 'phone' entry
unless ($phone) {
  $this->stash->{exception} .= "bad phone\n";
}

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

...