(Don't worry about lockf: you want flock)
There are a few things we need to know:

• What system is this on?
• Are you opening it with write access?
• How do you know that the lock is succeeding?
• How do you know it is not locking the file?
• Does $! tell you anything?
• Is this a cgi script?

Things to try:

• Make sure your system supports file locking!
• Check the return values of everything
• Open with explicit read/write access
• Try shared and exclusive locks

Here's simple code for a lock:

open(LOCKME, "+< $file_to_lock") or die "No open $!\n";
flock(LOCKME, LOCK_EX) or die "No lock $!\n";
sleep(20); ## Have another process test the lock now...
close(LOCKME) or die "No close $!\n";
[download]

Note that close dissolves the lock for you, so you don't
need to use flock to unlock it.