I don't want to protect $x. I just want to touch $x (store the value into it) when Scope::Guard object gets destroyed, i.e. when all references to it go out of scope.

In simple and obvious example below, the $g gets destroyed and inner sub called immediately after it goes out of scope.

use strict;
use Scope::Guard;

my $x;
{
   my $g = Scope::Guard->new(sub {$x; warn "destroyed"});
}
warn "end";
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But I checked, and putting braces around the dcl of $x, doesn't change the output. So I would bet scopeguard has stored a reference to the interior sub so it can be called when "$x" goes out of scope.

Your bet is wrong. The sub is called the guard gets destroyed, not the object it references. Look at the Scope::Guard code. Is very short and self explanatory.

All you did by assigning undef to $code is lose the reference to the outside sub.. SGnew still has a reference to the inner sub and $x...

Yes, but when I lose reference to the outside sub, I should also lose the last reference to $g, because the sub is the only object which sees $g. But it doesnot happen.

Does that make more sense?

I need to read comments above, but it still doesnot make more sense to me. However constructed is the inner sub (stored in $g) it is just another object referenced by $g, not referencing it. So I don't understand how it (the inner sub) can affect the reference count to $g.

The example I gave doesnot make sense itself, it is just the simplest demonstration of the behaviour I was able to make up. My original was more like the test below: ($x is \@res, I just try to log into res, when the object gets destroyed).

use strict;
use Test::More tests=>2;
use Scope::Guard;

sub try {
    my @res;
    my @subs = do {
        my $g;
        (   sub {
                push @res, 'a';
                $g = Scope::Guard->new(
                    sub {
                        warn "destroyed";
                        push @res, 'destroyed'
                    } );
            },
            sub {
                push @res, 'b';
                undef $g;
            }
        );
    };
    ( shift @subs )->();
    is_deeply( \@res, ['a'] );
    undef @subs;
    is_deeply( \@res, [ 'a', 'destroyed' ] );
}

try();
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