#GET CURRENT NUMBER
open(FH, "counter.dat");
 flock(FH, 1);
 $id = <FH>;
close(FH);

#INCREASE COUNT
$id++;

#WRITE NUMBER BACK
open(FH, "+< counter.dat");
 flock(FH, 2);
 seek(FH, 0, 0); truncate(FH, 0);
 print FH $id;
close(FH);