When to use .new?

[ToddAndMargo via perl6-users]

Hi All,

Why can I get away with

   my Str $x = "";

But I have to use .new here (an other places too)

   my $ppSession = NativeCall::Types::Pointer.new();


Is there some rule I can follow that let me know
when I have to use .new and when I do not?
(I am getting tired of figuring it out the
hard way.)

Many thanks,
-T