use strict;
use warnings;

warn "Doesn't Exist!" unless exists &CORE::GLOBAL::sleep;
*CORE::GLOBAL::sleep =sub {42};
warn "Exists!" if exists &CORE::GLOBAL::sleep;

##</code><code>##

Doesn't Exist! at d:/Users/lanx/pm/core_sleep.pl line 4.
Exists! at d:/Users/lanx/pm/core_sleep.pl line 6.