open T1, ">>test1.out";
open T2, ">>test1.out";
foreach (0..20) {
 print T1 "1:$_$/";
 print T2 "2:$_$/";
}
close T2; #notice close order...
close T1;
open T1, ">>test1.out";
open T2, ">>test1.out";
select T1;
$|=1;
select T2;
$|=1;
foreach (0..20) {
 print T1 "3:$_$/";
 print T2 "4:$_$/";
}
close T2;
close T1;