When I need to share authorization code between different C::A applications, I put the necessary code in the cgiapp_prerun() method of a base module; then every application use that module and inherits everything needed, including some common run modes (i.e. login, login_failed, etc). BTW, this is also the suggested solution in CGI::Application documentation.
The following is a stripped down example from my code:

package MyApp::Base;
use base 'CGI::Application';
sub cgiapp_init {
  my $self = shift;

  # init your application: session, database, whatever is needed
  ...

  # set the name of the run mode CGI param
  $self->mode_param('rm');

  # shared run modes
  $self->run_modes('login'    => 'login',
                   'AUTOLOAD' => 'autoload_exception');

  return;
}

...
sub session { return shift->param('SESSION'); }
...

sub cgiapp_prerun {
  my $self = shift;
  my $runmode = shift;
  my $user = $self->session->param('user');
  unless ($user) {
    $self->prerun_mode( 'login' );
  }
}
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package MyApp::Application1;
use base 'MyApp::Base';
...
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