The shallowness of .clone needs a more precise explanation.

[smls]

objects#Object_Cloning says:

This cloning is shallow since it only binds attributes to the same values contained in the original object; it does not make copies of those values.

I don't think that's completely accurate.

A $. attribute does get a fresh Scalar container, and the value from the old object's attribute gets assigned into it:

class A { has $.x is rw }

my $a = A.new(x => 2);
my $b = $a.clone;

$b.x = 10;

say $a;  # A.new(x => 2)
say $b;  # B.new(x => 10)

...whereas a @. attributes doesn't get a fresh Array container, but rather gets the Array from the old object's attribute bound to it.

So it's more like how arguments are passed to $ and @ parameters in a signature, than simple := binding.
Not sure how to properly word that for the docs.