The following example can help you.
The main process forks a number of children with logical IDs between 0 and $np. The processes concurrently gain exclusive access to a file using a lock.

lhp@nereida:~/Lperl/src/perl_networking/ch2$ cat -n flock.pl
 1  #!/usr/bin/perl -w
 2  use strict;
 3  use Fcntl qw(:DEFAULT :flock);
 4  use POSIX qw(WNOHANG);
 5  $| = 1;
 6
 7  local $SIG{CHLD} = sub {
 8    while (my $kid = waitpid(-1, WNOHANG) > 0) {}
 9  };
10
11  sub create_child {
12    my ($id, $task) = splice @_, 0, 2;
13    my $pid;
14
15    return $pid if $pid = fork();
16    die "Cannot fork $!" unless defined $pid;
17    $task->($id, @_); # do something
18    exit;
19  }
20
21  sub parfor {
22    my $LAST = shift;
23    my $task = shift;
24    my @pid;
25
26    $pid[0] = $$;
27    $pid[$_] = create_child($_, $task, @_) for 1..$LAST;
28    return @pid;
29  }
30
31  sub task {
32    my $id = shift;
33    my $fn = shift;
34
35    sleep(int(rand(2)));
36    open my $f, "+<$fn" or die "Can't open $fn. $!\n";
37    flock($f, 2);
38    seek $f, 0, 0;
39    my $num = <$f>;
40    warn "Process $id reads $num.\n";
41    seek $f, 0, 0;
42    my $s = $num+$id;
43    warn "Process $id writing $s.\n";
44    print $f $s;
45    close($f);
46    exit;
47  }
48
49  #main
50  my $np = shift || 3;
51  my $fn = shift || "sync.txt";
52  open my $f, "> $fn" or die "Can't open file $fn. $!\n";
53  print $f 0;
54  close($f);
55  &parfor($np, \&task, $fn);
56  do {} while wait > 0;
[download]

The answer to your question is in line 56

Hope it helps

Casiano