open(FH, "+</etc/book") or die "$0 can't open the books\n";
flock(FH,2);
# the file is now locked, any processes trying
# to open this file will wait until I am finished
# notice the arg '2' . . . this means exclusive lock

# this area between the lock and the unlock is
# called the critical section - keep your code
# short, sweet, and fast as possible to prevent
# the file from being locked for too long

# do stuff
close(FH);   # in Perl, close() will unlock your file