Form Validator for value in responseJSON

I've been trying to figure this out for weeks, but I keep running into dead ends. I'm using Perch Shop, and I've created a code generator that creates a code at checkout, and associates it with a given order. This data is then stored in a form, using the perch:form functionality. I want to then allow a user to enter this code, and have perch validate its existence from the responseJSON column of the perch2_forms_responses table. I've created a validator class for this.

class Perch_Forms_Validators
{
    public static function check_promocode($text)
    {
        $API = new PerchAPI(1.0, 'perch_forms');
        $db     = $API->get('DB');

        $sql = 'SELECT COUNT(*) FROM perch2_forms_responses WHERE responseJSON='.$db->pdb($text);
        $count = $db->get_count($sql);

        if ($count > 0) {
            return true;
        }

        return false;
    }
}

I recognize that the above code with reference the entire JSON string. I'm trying to figure out if there is any way to reference a specific value within the JSON data. Can I somehow use

decode_json(responseJSON, true);

I'm using version 5.6.33 of MySQL, so there is no native JSON functionality.