# before the loop
my $sth1 = $dbh->prepare("select d_id, c_id from d where d_id = ?");
        
my $sth2 = $dbh->prepare("select d_id, c_id from d where d_id = ?");

# Then within the loop
$sth1->execute($rec{did1});
$sth2->execute($rec{did2});