package AB;
sub testing {
    print $_[0] . ": " . __PACKAGE__ . "\n";
}
1;


package CD;
sub testing {
    print $_[0] . ": " . __PACKAGE__ . "\n";
}
1;

package main;

use strict;
use warnings;
use 5.014;

my $count = 0;
foreach ( qw/ AB CD/ ) {
    no strict 'refs';
    &{$_ . '::testing'}($count++);
}

*testing = \&AB::testing; 
# redefined warning
testing($count++);

*testing = \&CD::testing; 
# redefined warning
testing($count++);

$::{testing} = \&AB::testing; 
# redefined warning
testing($count++);

$main::{testing} = \&CD::testing; 
# redefined warning
testing($count++);

$main::{testing} = *AB::testing{CODE};  
# redefined warning
testing($count++);

*testing = *CD::testing;
testing($count++);

*main::testing = *AB::testing;
testing($count++);

$main::{testing}= *CD::testing;
testing($count++);

foreach( qw/AB CD/ ) {
    no strict 'refs';
    *main::testing = *{ $_ . "::testing" };
    testing($count++);
}