LOCK_EX | LOCK_NB

gistfile1.pl

use Fcntl qw(:flock LOCK_NB);
use Parallel::ForkManager;

my $pm = new Parallel::ForkManager(2);

for (1..2) {
    my $pid = $pm->start and next; 
    write_file();
    $pm->finish;
}
$pm->wait_all_children;

sub write_file {
    open (my $fh, '>', './touch') or die $!;

    my $start = time;
    my $locked = flock($fh, LOCK_EX|LOCK_NB);
    sleep 5;
    flock($fh, LOCK_UN);
    my $end   = time;

    warn sprintf("proc(%d) - locked: %s, waited time: %d", $$, $locked ? 'true' : 'false', $end - $start);

    close $fh;
}