use v5.12;

# This is basically a dummy override that must be performed early
# on so that Perl knows we'll be overriding CORE::GLOBAL::caller
# later...
#
BEGIN { *CORE::GLOBAL::caller = sub { CORE::caller(@_) } };

sub foo {
	say scalar caller;
}

sub bar {
	local *CORE::GLOBAL::caller = sub { 'xxxx' };
	foo();
}

foo();
bar();

##</code><code>##

$ perl -MO=Concise -E'BEGIN { *DUMMY::GLOBAL::caller = sub {} }; caller()'
4  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 49 -e:1) v:%,{,2048 ->3
3     <0> caller[t1] v* ->4
-e syntax OK
$ perl -MO=Concise -E'BEGIN { *CORE::GLOBAL::caller = sub {} }; caller()'
6  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 49 -e:1) v:%,{,2048 ->3
5     <1> entersub[t3] vKS/TARG,1 ->6
-        <1> ex-list K ->5
3           <0> pushmark s ->4
-           <1> ex-rv2cv sK ->-
4              <#> gv[*CORE::GLOBAL::caller] s ->5
-e syntax OK