The addition of threads has changed Perl's internals substantially. There are implications for people who write modules - especially modules with XS code or external libraries. While most modules won't encounter any problems, modules that aren't explicitly tagged as thread-safe should be tested before being used in production code.

Not all modules that you might use are thread-safe, and you should always assume a module is unsafe unless the documentation says otherwise. This includes modules that are distributed as part of the core. Threads are a beta feature, and even some of the standard modules aren't thread-safe.

If you're using a module that's not thread-safe for some reason, you can protect yourself by using semaphores and lots of programming discipline to control access to the module. Semaphores are covered in the article.