I had a similar problem yesterday (and found this thread while doing a bit of research) but found an alternative solution in the module docs and thought I'd write it up here for the next time someone searches.

From the docs:

Multiple constraints can be applied to a single field by defining the value of the constraint to be an array reference.

I needed to (sort of) check the validity of an email address and make sure that the address' owner was not already registered.

To complicate matters a bit further, I needed to check that the entry in a field to confirm the email address matched.

Here's an abridged version of the form profile I used. It presupposes the existence of a Class::DBI derived package that knows about users:

my $profile = {

  required      => [ qw(

    user_name
    user_email
    user_email_confirm

    # more stuff here...

  ) ],

  msgs          => {

    prefix      => q[e_],
    constraints => {

      # helpful error messages:

      user_email_valid  => q[Invalid address],
      user_email_lookup => q[Already registered],
      emails_match      => q[Emails don't match],
    },
  },

  # tidy entries:

  filters       => ['trim','strip'],

  constraints   => {

    user_email            => [

      {
       name       => q[user_email_valid],
       constraint => q[email],
      },
      {
       name       => q[user_email_lookup],
       constraint => sub {

         # have a look in database:

         !My::Content::User->search(

           user_email => $_[0],
         );
       },
       params     => [ qw(

         user_email
       ) ],
      }
    ],

    user_email_confirm    => {

      name       => q[emails_match],
      constraint => sub { $_[0] eq $_[1] },
      params     => [ qw(

        user_email
        user_email_confirm
      ) ],
    },

    # more constraints, perhaps...
  },
};
[download]

HTH someone,