use strict;
use warnings;
use threads;
use Fcntl qw(:flock);

open(FILE, ">>/tmp/test.txt");

for(0..9) {
    threads->new(\&worker, $_);
}

$_->join for threads->list;

sub worker {
    my $x = shift;

    flock(FILE, LOCK_EX);
    print "$x: locked file\n";
    sleep(10);
    flock(FILE, LOCK_UN);
    print "$x: unlocked file\n";
    return;
}

##</code><code>##

0: locked file
1: locked file
2: locked file
3: locked file
4: locked file
5: locked file
6: locked file
7: locked file
8: locked file
9: locked file
0: unlocked file
1: unlocked file
2: unlocked file
3: unlocked file
4: unlocked file
5: unlocked file
6: unlocked file
7: unlocked file
8: unlocked file
9: unlocked file