What your doing here is essentially a shell script with a Perl wrapping. I can appreciate that in the beginning you might be tempted to do this. But if this is what your going to do you may as well leave Perl out of it. If you have an immeadiate, one time need to untar a group of files you may be better off just using shell, but if you truly want to use Perl because it's called for then think about the best practices to implement your needs. Perl is a great tool for doing all kinds of programming, whether it's a system task or database system or whatever but you shouldn't go out of your way to do it in Perl, especially if your going to just do `` and system calls. In this example that's already been pointed out you could use Archive::Tar and that would be usefull if your application was doing more than just untaring, and certainly if this is a practice script to get your feet wet with Perl it's a good way to go. But if your really just trying to get your work done I'd suggest using the most pratical tool for the job. Perl one-liners, shell scripts, c, c++ whatever is best. You might be able to get a screw out using a circular saw but it may be a little overkill.
Good luck and come back often as you move up the learning curve. The Perlmonks is a great resource!