warn $];
warn \&CORE::GLOBAL::sleep;
warn \&CORE::sleep;
&CORE::GLOBAL::sleep(3);

##</code><code>##

5.016003 at d:/Users/lanx/pm/core_sleep.pl line 1.
CODE(0x328420) at d:/Users/lanx/pm/core_sleep.pl line 4.
CODE(0x349718) at d:/Users/lanx/pm/core_sleep.pl line 5.  <-- different!
Undefined subroutine &CORE::GLOBAL::sleep called at d:/Users/lanx/pm/core_sleep.pl line 7.

##</code><code>##

*old = *CORE::GLOBAL::sleep;
*CORE::GLOBAL::sleep = sub { warn " **** beware of morons **** "};

&old();                                 # oops
warn \&CORE::GLOBAL::sleep;
warn \&old;

##</code><code>##

 **** beware of morons **** at d:/Users/lanx/pm/core_sleep.pl line 2.
CODE(0x4c9478) at d:/Users/lanx/pm/core_sleep.pl line 7.
CODE(0x4c9478) at d:/Users/lanx/pm/core_sleep.pl line 8. <-- identical!

##</code><code>##

lanx@ubuntu14-large:~$ perl
warn $];
warn \&CORE::GLOBAL::sleep;
warn \&CORE::sleep;
&CORE::GLOBAL::sleep(3);
__END__
5.018002 at - line 1.
CODE(0x1804cb8) at - line 2.
CODE(0x1830398) at - line 3.
Undefined subroutine &CORE::GLOBAL::sleep called at - line 4.

##</code><code>##

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.