There is something I don't understand about your question. Do you need multitasking on the OS level, or on the Perl level ?

If you have a program (be it an executable or a Perl script run by the Perl interpreter), you can run multiple instances of it on all the common OSes. Common OSes also have schedulers, "cron" on Unixes, and "at" (or "System tools" -> Scheduled tasks) on Windows.

On the other hand, if you want to run multiple instances of some task from your Perl script, the other monks gave you good ideas - basically your options are processes or threads. What suits you most depends on the specifics of your application.

I'm sure that if you post a clarification telling exactly what you want done, you will get very good answers.