% reply
0> use Set
1> my $s = Set->new(_items => [1 .. 4])
$res[0] = bless( {
         '_items' => [
                       1,
                       2,
                       3,
                       4
                     ]
       }, 'Set' )

2> $s->add(5)
Not a HASH reference at Set.pm line 15.