package MyCGIApp::Runmode1;

use strict;
use Runmode1specficmodules;

sub do_runmode1 {

  my $self = shift;
  my $session = $self->session;
  my $q = $self->query();
  my $id = $q->param('n');

  unless( somecondition ) return $self->error;
 
 ...do stuff...

  return $html_output;

}

sub error {

  ...runmode1 error stuff...

}

1;