use strict;
use warnings;
use Fcntl qw( :flock );
sleep(1);
open(my $fh, '+<', $0) or die "open: $!\n";
print localtime().": $$'s flock attempt starts\n";
flock($fh, LOCK_EX) or die "flock: $!\n";
print localtime().": $$'s flock obtained\n";
sleep(5);
print localtime().": $$ ends\n";