Variables: type constraints, default values.

Kristien++ pointed out the lack of docs

lib/Language/variables.pod

@@ -451,6 +451,30 @@ say $cookies; # -> 6
 Like the L<C<temp>|#The_temp_Listop> listop, but only restores the
 value if the block exits unsuccessfully.
 
+=head1 Type Constraints and Initialization
+
+Variables can have a type constraint, which goes between the declarator and
+the variable name:
+
+=begin code :allow<L>
+    my Int $x = 42;
+    $x = 'a string';    # throws an L<X::TypeCheck::Assignment|/type/X::TypeCheck::Assignment> error
+=end code
+
+If a scalar variable has a type constraint but no initial value, it is initialized
+with the type object of the constraint type.
+
+    my Int $x;
+    say $x.^name;       # Int
+    say $x.defined;     # False
+
+Scalar variables without an explicit type constraint are typed as
+L<Mu|/type/Mu>, but default to the L<Any|/type/Any> type object.
+
+Variables with the C<@> sigil are initialized with an empty
+L<Array|/type/Array>, Variables with the C<%> sigil are initialized with an
+empt L<Hash|/type/Hash>.
+
 =head1 Special Variables
 
 Perl 5 is infamous for its many obscure special variables. Perl 6 also