no warnings 'redefine';
my $real_new;
BEGIN { $real_new = Foo->can( 'new' ) }
*Foo::new = sub {
    if (caller() eq 'Bar') {
        print "Returning real Foo.\n";
        return Foo->$real_new();
    }
    print "Returning fake Foo.\n";
    return FakeFoo->new;
};