use Fcntl ':flock';
use POSIX ':sys_wait_h';

my $fh;
open($fh, 'somefile.txt');
flock($fh, LOCK_EX);
my $pid = fork();
if ($pid == 0){
     #this is the child...does it have the lock?
     
     #do some stuff....

     exit;
} elsif (!defined($pid)) {
    die "Couldn't fork: $!";
} else {
    #this is the parent...does it have the lock?
    waitpid($pid, &WNOHANG);
}