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my $got_lock;
use Fcntl;   # to get constants for O_CREAT | O_EXCL | O_RDWR
...
    sleep 2;
}
diehtml("Lock error $!\n") unless $got_lock;

system ("lockfile -2 -r 5 $base_dir/.lock")  == 0
        or diehtml("Lock error: ", $? >> 8, "\n");
        # TODO stop stderr of system