use DBI;
my $hour_db = DBI->connect("DBI:mysql:database=$db;host=$host;mysql_socket=/opt/lampstack-5.5.27-0/mysql/tmp/mysql.sock","root","", {'RaiseError' => 1});
my @column_names = ("MO_resp","MT_resp","AO_resp");
foreach my $column_name (@column_names)
{
    my $hour_sth = $hour_db->prepare("Select sum($column_name) from $table_name where Date='$db_date' and Hour like='$hour:%'");
    $hour_sth->execute() or die $DBI::errstr;
    .....    
}