+The level of the C<$stack>-th upper subroutine/eval/format context.
+It kind of corresponds to the context represented by C<caller $stack>, but while e.g. C<caller 0> refers to the caller context, C<CALLER 0> will refer to the top scope in the current context.
+For example,
+
+ reap ... => CALLER(0)
+
+will fire the destructor when the current subroutine/eval/format ends.