#WRITE NUMBER BACK
open(FH, "+< counter.dat");

##</code><code>##

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