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;
}