lanx@ubuntu14-large:~$ perl
*old = *CORE::GLOBAL::sleep;
*CORE::GLOBAL::sleep = sub { warn " **** beware of morons **** "};

&old();                                 # oops
warn \&CORE::GLOBAL::sleep;
warn \&old;
__END__
 **** beware of morons ****  at - line 2.
CODE(0x2089448) at - line 5.
CODE(0x2089448) at - line 6.