my $ca_obj = shift; # CGI::Application object
my $q = $ca_obj->query();

my $dfv_profile =
{
    required	=> [qw (userid)],
    constraints	=>
    {
      userid =>
      {
          name 	     => 'userid_exists',
          constraint => sub { 
                return !MyDBITools->user_exists($ca_obj); },
      },
      userid =>
      {
          name	     => 'userid_untaint',
          constraint => qr/^[-\@\w.]+$/,
      },
    },
    msgs =>
    {
        format	    => '%s',
        constraints =>
        {
            'userid_exists' => "User ID already exists.",
            'userid_untaint'=> "User ID contains bad stuff",
        },
    },
};
			
my $results = Data::FormValidator->check($q, $dfv_profile);