Not all databases like to have two active statements at the same time. For this type of problem, I find it better to create two connections to the database.

my $db1= first connection
my $db2= second connection
...
my $sth = $db1->prepare("SELECT constraint_name FROM information_schem
+a.constraint_column_usage WHERE TABLE_NAME = '$tbl'");
$sth->execute;
my $temp =  $sth->fetchrow_arrayref;
foreach my $rel (@$temp) {
    next unless ($rel =~ /ID/);
    print "Relationship: $rel\n";
    my $th = $db2->prepare("ALTER TABLE Data_CentersTEST DROP CONSTRAI
+NT $rel");
    $th->execute;
}
[download]