I was wondering if I am doing something wrong

There really is no way to tell from the sub selection of the code you've chosen to post.

You mention "thread", but there is nothing to indicate whether you mean threads, or if that's a euphamism for fork, or a simply a colloqualism meaning a new client.

There is also mention of "Clone()" in the comments, which could be significant--or not.

The only thing that stands out as being worthy of closer scrutiny is:open(DEBUG,">$DEBUG_LOG");

Re-opening the same glob filehandle, never checking for success, and never closing it, in a concurrent environment, might or might not be a problem.