You do use flock, which is good, but if the file is opened while another instance is in the for()-loop, the counter will in effect only be increased once. To avoid this race condition, you have to write to the file without closeing it after reading. This can be done using a read+write mode.

sub counter {
    my ($file) = @_;
    local *COUNTER;
    local $/ = undef;
    open           COUNTER, '+<', $file or
    open           COUNTER, '>',  $file or return undef;
    flock          COUNTER, 2;
    seek           COUNTER, 0, 0;
    my $counter = <COUNTER>;
    seek           COUNTER, 0, 0;
    truncate       COUNTER, 0;
    print          COUNTER ++$counter;
    close          COUNTER;
    return $counter;
}
[download]

There are also CPAN modules that do this, but I think modules are overkill for a ++.

- Yes, I reinvent wheels.
- Spam: Visit eurotraQ.