I would think that's still a closure. Since it is called outside of the lexical scope of the lexical variable. When a signal is caught, we're in the signal handling part of the perl runtime. That is far, far outside of the lexical scope of $warn. So, Perl_warn_handler() calls the coderef it is given, which affects $warn, which is outside of the scope of Perl_warn_handler(). That, to me, is a closure.