$q->param( userid_untaint => 'dummy junk' );

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