sub display_form {
   ...
}

sub process_form {
   my $self = shift;

   my @input = data_from_query_or_session();

   # use CAP::ValidateRM here
   my @errors = validate( $profile, @input );

   if( @errors ) {
      # HTML::FillInForm makes this easier
      return $self->display_form( @errors );
   }

   # submission was ok, process the data
   ...
}