sub filelock;

filelock "lockit";

Do the inplace edit stuff here

filelock "unlock";

sub filelock {

 if ( @_ eq "lockit" ) {
         open(LCK,">abc.lock") or die "Can't open abc.lock: $! \n";
         unless (flock LCK, LOCK_EX) {
                warn "Waiting to obtain lock on LCK ....\n";
                flock(LCK, LOCK_EX) or die "Cannot lock semaphore file - $!\n";
         }
 }

 if ( @_ eq "unlock" ) {
        close(LCK)
 }

}