This is an interesting problem indeed.
I have used this method of opening the script itself and placing an exclusive lock on it for quite sometime, without problems

However, I have started working with Unix AIX recently and the solution no longer works :(

I had never had any problems with several Linux distros, FreeBSD, Mac OS X or Windows.

But sadly under AIX the script fails to get a lock on itself, weather it is running or not.

I added a $! to the flock like so:

flock LH, LOCK_EX|LOCK_NB
            or die "$0 is already running somewhere!\n$!";
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and the OS error message is:
A file descriptor does not refer to an open file

Interesting, the error message reads as if the file open failed,
and yet it was fine as far as I can tell because the 'or die' for open did not get invoked... odd.

Any suggestions?