For any individual instance of a running script Perl executes statements serially so if you have only one copy of the script running there should be no problem.

You could of course easily run multiple instances of the same script either on one computer, or even on several different computers that share access to the common file. If such a thing is possible there are many ways to manage access to your common file. Correctly handling multiple access to a common file can be very difficult though, depending somewhat on what you are actually trying to do. If you think it is possible that several copies of the script could be running at the same time you should describe what you are doing and seek further advice.