Unfortuantely this requires code to call the clone method, so won't work for me.

I'm probably just as confused as you are, but I had been thinking that, short of modifying the macros, that something like this might help:

#!/usr/bin/perl -slw

use strict;
use warnings;
use B;
use Clone::Fast;
use Object::Destroyer;

my $ref = {}; 
print B::svref_2object($ref)->REFCNT; 

my $copy = Clone::Fast::clone($ref);
print B::svref_2object($ref)->REFCNT; 

my $sentry = Object::Destroyer->new(sub { undef $copy });
print B::svref_2object($ref)->REFCNT;
[download]

Then I ran it using Devel::FindRef:

#!/usr/bin/perl

use strict;
use B;
use Clone::Fast;
use Devel::FindRef;
use Scalar::Util;
use Object::Destroyer;

our $ref = {};
my $global_my = \$ref;
our(%global_hash) = (ukukey => \$ref);
our(%global_hashref) = {ukukey2 => \$ref};

sub testsub {
    my $testsub_local = my $global_hashref;
    print Devel::FindRef::track(\$ref);
}

my $closure = sub {
    my $closure_var = \$_[0];
    Scalar::Util::weaken (my $weak_ref = \$ref);
    testsub;
};

sub my_ref {
    my $ref = {}; 
    print B::svref_2object($ref)->REFCNT;

    my $copy = Clone::Fast::clone($ref);
    print B::svref_2object($ref)->REFCNT; 

    my $sentry = Object::Destroyer->new(sub {undef $copy});
    print B::svref_2object($ref)->REFCNT; 
    testsub;
}
$closure->($ref);
[download]