In your example, here is what happens:

• at line 7 a closure is created that keeps references to $x and $g
• at line 10 ("$code->()"), the Scope::Guard object is instantiated and stored in variable $g, with a new closure referencing $x (or the reference of $x referenced in the closure $code).
• when $code is undef, it looks like that the closure can not be destroyed because of a circular dependency. It seems that the reference to $x kept is not the global one but the indirect one through the enclosing closure. I think this is a bug in perl.

$g is never released until global destruction. So the Scope::Guard object is never called until that time. This is one problem in your code.

The difference between when you put $x or not is that in the first case it is a closure that keeps a reference to $x while in the second case this is just a bare anonymous sub. In the second case there is no circular reference.

You should also try to call $code->() multiple times to see when the Scope::Guard object is destroyed (because the content of $g is replaced).

You can explore more what happens to references by using Devel::Refcount:

use strict;
use Scope::Guard;
use Devel::Refcount 'refcount';

sub showcount ($\$)
{
    printf "line %d: \$%s => %d\n", (caller)[2], $_[0], refcount($_[1]
+)-1;
}

my $x;

showcount(x => $x);

my $code = do {
    my $g;
    showcount(g => $g);
    # By creating a closure that reference $g, we increase its refcoun
+t to 2
    sub {
        # Here the only reference to $g is the closure
    showcount(g => $g);
    $g = Scope::Guard->new( sub { warn "destroyed"; $x; } );
    }
    # Here $g is out of scope, so refcount of $g decreases to 1
};

showcount(x => $x);
$code->();
showcount(x => $x);
#$code->();
showcount(x => $x);
undef $code;
# Here, I expect the refcount of $x to decrease, but it doesn't
showcount(x => $x);
warn "end";
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