+ The level of the $stack-th upper subroutine/eval/format context. It kind
+ of corresponds to the context represented by "caller $stack", but while
+ e.g. "caller 0" refers to the caller context, "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.