sub mock {
    my $thing = shift;
    my $self;

    if (ref($thing) eq __PACKAGE__){
        $self = $thing;
    }
    else {
        $self = bless {}, __PACKAGE__;
    }
    my $closure_self = $self;
    use Scalar::Util 'weaken';
    weaken $closure_self;

    $self->{sub} = \&One::foo;
    *One::foo = sub { $closure_self->{x} = 'x'; return "baz\n"; };
    return $self;
}

##</code><code>##

foo
baz
destroying...
foo

##</code><code>##

print One::foo();
{
    #my $count = Count->new;
    my $bar = Count->mock;
    print One::foo();
}
print One::foo();

##</code><code>##

foo
Use of uninitialized value in subroutine dereference at script.pl line 31.
destroying...
baz
baz
	(in cleanup) Unable to create sub named "" at script.pl line 31.