#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11148582
use warnings;
use Time::HiRes qw( sleep );

sub grabthelockorexit
  {
  my $lockfilename = '/tmp/d.11148582.lockfile'; # FIXME to your filename
  open our $fh, '>>', $lockfilename or die "$! on $lockfilename";
  use Fcntl qw(:flock);
  flock $fh, LOCK_EX | LOCK_NB or die "$$ exiting $!\n";
  }

# The following is just test code

for (1 .. 9)
  {
  if( my $pid = fork )
    {
    sleep 0.33;
    }
  elsif( defined $pid  )
    {
    sleep 0.1 + rand( 0.1 );

    grabthelockorexit();

    print "$$ got lock\n";
    sleep 1; # FIXME body of code...
    print "$$ exiting\n";
    exit;
    }
  else { die "fork failed" }
  }
1 while wait > 0; # wait until all children finish