my @threads;
for my $i (1..2) {
  push @threads, threads->create(\&testsub, $dbh->clone());
}

foreach my $thread (@threads) {
    $thread->join();
}

sub testsub
{
  for my $key (@{$_[0]->selectcol_arrayref("SELECT id, sleep(0.1) FROM keywords WHERE 1")}) {
        print $key;
  }
}