Somehow I hadn't grokked that in the scenario that $thing is a Count it gets cached in the sub. That is a problem solvable via weaken by saying 
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;
}

[download]
which outputs 
foo
baz
destroying...
foo

[download]
as per the spec. Note that the $closure_self misdirect is necessary because of the Class method invocation: 
print One::foo();
{
    #my $count = Count->new;
    my $bar = Count->mock;
    print One::foo();
}
print One::foo();

[download]
which would yield 
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.

[download]
Because $self immediately goes out of scope. It's still a weird misdirection since you are localizing a subroutine clobber and not changing an object behavior.

#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.

In reply to Re^3: Trying to DESTROY() an object by kennethk
in thread SOLVED: Trying to DESTROY() a (closure-wrapped) object by stevieb

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