constraints => {
    customer_name => {
        params => [ qw/ customer_name email age / ],
        constraint => sub {
            my ($name, $email, $age) = @_;
            # untaint, trim, whatever
            return My::Customer->find_or_create( ... );
        }
    }
}

##</code><code>##

package Customer::Handler;

sub new {
    my ( $class, $cgi ) = @_;
    my ( $name, $email, $age ) = 
      map { scalar $cgi->param($_) } qw/customer_name email age/;
    # untaint, trim, whatever
    return My::Customer->find_or_create( ... );
}

1;