use Fcntl qw(:flock);
use Symbol;

fork(); # child and parent do the same operation

my $fh = gensym;
open $fh, ">/tmp/lock" or die "cannot open /tmp/lock: $!";
print "$$: I'm going to obtain the lock\n";
flock $fh, LOCK_EX;
print "$$: I've got the lock\n";
sleep 30;
close $fh;