not a threads expert, but it looks to me like on Win32, sharing is acheved by cloning the object several times to pass to different threads.. so if you try:

use threads;
use threads::shared;
{
    package A;
    use threads;
    use threads::shared;
    sub new {
        my $class = shift;
        my $self = &share({});
          print "Created $class at $self\n";
        return bless $self, $class;
    }
    sub get_s {
    }
    sub DESTROY {
        print "Destroy $_[0]\n";
    }
}

my $a :shared;
$a  = A->new();
for (1..2) {
    async {
        print "thread start\n";
        $a->get_s();
    }->join();
}
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you'll see that only the last DESTROY is from an object created by 'new'..

P:\bin>perl threads_test.cgi
Created A at HASH(0x225800)
thread start
Destroy A=HASH(0x1e07d9c)
thread start
Destroy A=HASH(0x1dbba9c)
Destroy A=HASH(0x225800)
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Try an inside-out approach:
in 'new', register the created $self in a static hash( ie,

package A;
our %created_objs : unique;

sub new {
...
bless $self, $class;
$created_objs{ $self } = 1;
return $self;
}
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and then in DESTROY first check that your object is an original :

sub DESTROY {
    my $self = shift;
    if ($created_objs{$self}) {
        # do my DESTROY stuff
        delete $created_objs{ $self };
    }
}
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update: added a little code to show that registering must happen after the blessing
update 2: to be perfectly safe, you probably want instead to have a private attribute which is your key in the static hash.. otherwise if your object gets reblessed, you're screwed - "$self" will change