#!/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

##</code><code>##

79504 got lock
79505 exiting Resource temporarily unavailable
79506 exiting Resource temporarily unavailable
79509 exiting Resource temporarily unavailable
79504 exiting
79510 got lock
79511 exiting Resource temporarily unavailable
79514 exiting Resource temporarily unavailable
79510 exiting
79515 got lock
79516 exiting Resource temporarily unavailable
79515 exiting