use Fcntl ':flock';
flock(file,LOCK_EX);
seek file, 0, 0;

##</code><code>##

#!/usr/bin/perl -wT
use Fcntl qw(:flock);
my $file = 'test_lock.txt';
my $SEMAPHORE = $file . '.lck';
open(S, ">$SEMAPHORE") or die "$SEMAPHORE: $!";
flock(S, LOCK_EX) or die "flock() failed for $SEMAPHORE: $!";
open (FH, ">>$file") or die "Can't open $file: $!";
print "About to write\n";
print FH "I have written ($$)\n";
print "Written\n";
close FH;
print "Going to sleep...\n";
sleep 10;
print "Woken up...\n";
close S;