#Lock my file...

# Use LOCK_SH because I don't want the second thread to be able 
# to read until I'm done.
flock( OUTFILE, LOCK_SH);  
seek( OUTFILE, 0, 2); # Why do I need this line?

# Read from OUTFILE and if it has the right value then go ahead and
# write the new value into it to indicate that the second process
# should do something different, otherwise you are the second
# process... unlock and do something different.

# Now unlock...
flock( OUTFILE, LOCK_UN);