Regarding the hash copied in each thread. this is the normal behaviour with perl threads (Edit: As Anonymous Monk already said). But you can declare your hash my %hashNR :shared; and hopefully this will be enough. However your thread strategy can become much leaner, for example employ a pool of threads so you do not continually creating the threads, as Anonymous Monk said. The answer by BrowserUK will help you regarding a thread pool: How to create thread pool of ithreads