Well, it is more interesting to show when the closure stored in $code is effectively destroyed:

use strict;
use Scope::Guard;

my $x;

my $code = do {
    my $g;
    sub {
        my $z;
        $g = Scope::Guard->new( sub { warn "destroyed"; $z } );
    }
};

use Scalar::Util ();
Scalar::Util::weaken(my $code2 = $code);

$code->();
printf "code2: %d\n", !!(defined $code2);
undef $code;
printf "code2: %d\n", !!(defined $code2);
warn "end";
[download]

Output on perl 5.14.2:

code2: 1
code2: 1
end at x.pl line 21.
destroyed at x.pl line 10 during global destruction.
[download]

As shown above, the issue is independent of $x being global: here I'm using $z which is local to the enclosing closure and the behavior is the same: the closure pointed by $code is not freed.