warn $];
warn \&CORE::GLOBAL::sleep;
warn \&CORE::sleep;
&CORE::GLOBAL::sleep(3);
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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.  <-- differen
+t!
Undefined subroutine &CORE::GLOBAL::sleep called at d:/Users/lanx/pm/c
+ore_sleep.pl line 7.
[download]

You are trying to build a wrapper around an non-existent function.

You haven't told us which version you use, but I doubt that CORE::GLOBAL:: is populated by default.

What you originally tried, something like:

*old = *CORE::GLOBAL::sleep

is aliasing the two symbols (two symbols pointing to the same typeglob)

But after populating the code slot with *CORE::GLOBAL::sleep = sub{} both will point to the same sub.

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

&old();                                 # oops
warn \&CORE::GLOBAL::sleep;
warn \&old;
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 **** 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
+!
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That's why you get a circular call!

update

same with 5.18 on linux

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.
[download]

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.
[download]