#!perl
use strict;
use warnings;
use Test::More tests => 2;

# hide package Outer (a little stupidly)
sub my_caller {
    my $package = CORE::caller(@_);
    $package = 'main' if $package eq 'Outer';
    return $package;
}

# redefine both caller (which fails) and glob (which works)
*CORE::GLOBAL::caller = \&my_caller;
*CORE::GLOBAL::glob   = \&my_caller;

# actual tests
my ($got_caller, $got_glob);
Outer::outer();
is($got_caller, 'main', 'redefined caller in all packages');
is($got_glob,   'main', 'redefined glob in all packages');

{
    package Inner;
    sub inner {
        $got_caller = caller();
        $got_glob   = glob();
    }
}

{
    package Outer;

    sub outer { Inner::inner() }
}
__END__                                                          
1..2
Name "CORE::GLOBAL::caller" used only once: possible typo at test.t line 14.
Subroutine CORE::GLOBAL::glob redefined at test.t line 15.
not ok 1 - redefined caller in all packages
#   Failed test 'redefined caller in all packages'
#   at test.t line 20.
#          got: 'Outer'
#     expected: 'main'
ok 2 - redefined glob in all packages
# Looks like you failed 1 test of 2.