As of release 5 of Perl, assignment to C<$[> is treated as a compiler
directive, and cannot influence the behavior of any other file.
+(That's why you can only assign compile-time constants to it.)
Its use is highly discouraged.
+Note that, unlike other compile-time directives (such as L<strict>),
+assignment to $[ can be seen from outer lexical scopes in the same file.
+However, you can use local() on it to strictly bound its value to a
+lexical block.
+
=item $]
The version + patchlevel / 1000 of the Perl interpreter. This variable