If I'm not sure whether or not flock will work, I usually use a workaround by renaming the file I want to have access to. Of course this depends on rename being "atomic", meaning: There is no point in time while renaming, where both filenames exists and rename may be halted.

so my code looks something like this:

my $cntr="counterfile";
my $owncntr="counter.mine";
while (! rename $cntr,$owncntr) {
  # sleep some time to retry
  # or die if no more retries allowed
}
# do some work on file $owncntr
if (! rename $owncntr,$cntr ) die "Someone created a new $cntr file?";
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