-C<$_[2]> is a either an alias or a copy of the current key, and C<$_[3]> is an alias to the current element (i.e. the value).
-Because C<$_[2]> might be a copy, it is useless to try to change it or cast magic on it.
+When the variable for which the magic is invoked is an array or an hash, C<$_[2]> is a either an alias or a copy of the current key, and C<$_[3]> is an alias to the current element (i.e. the value).
+Since C<$_[2]> might be a copy, it is useless to try to change it or cast magic on it.
+
+Starting from perl 5.17.0, this magic can also be called for code references.
+In this case, C<$_[2]> is always C<undef> and C<$_[3]> is a reference to the cloned anonymous subroutine.