Returns the glob with the given C<name> and a defined subroutine or
C<NULL>. The glob lives in the given C<stash>, or in the stashes
-accessible via @ISA and @UNIVERSAL.
+accessible via @ISA and UNIVERSAL::.
The argument C<level> should be either 0 or -1. If C<level==0>, as a
side-effect creates a glob with the given C<name> in the given C<stash>