use Fcntl qw/:DEFAULT :flock/;
sub counter {
    my $file = shift;
    sysopen(COUNTER, $file, O_RDWR|O_CREAT) || warn $! && return;
    flock(COUNTER,LOCK_EX)                  || warn $! && return;
    my $count = <COUNTER>                   || 0;
    seek(COUNTER,0,0)                       || warn $! && return;
    truncate(COUNTER,0)                     || warn $! && return;
    print(COUNTER $count + 1, "\n")         || warn $! && return;
    close(COUNTER)                          || warn $! && return;
    return $count + 1;
}
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