this is a (stupid) counter i wrote for exercise years ago: maybe it could help you...

!/usr/bin/perl -w

use Fcntl qw(:flock);
my $filename = '/tmp/counter';

if(-e $filename) {
   open FH, "+< $filename" || die "cannot open: $!";
}
else {
  #first time...
  open FH, "+> $filename" || die "cannot open: $!";
}
lock(FH);
my $count = <FH> || 0;
$count++;
seek FH, 0,0;
print FH $count;
unlock(FH);

print "$count\n";

sub lock {
 my $fh = shift;
 flock $fh, LOCK_EX;
}
sub unlock {
 my $fh = shift;
 flock $fh, LOCK_UN;
}
[download]