MooseX::OneArgNew lets your constructor take a single argument, which will be translated into the value for a one-entry hashref. It is a parameterized role with two parameters: type The Moose type that the single argument must be for the one-arg form to work. This should be an existing type, and may be either a string type or a MooseX::Type. init_arg This is the string that will be used as the key for the hashref constructed from the one-arg call to new.