Thread safety of global variables in perl

Question

I have following questions:

1. How is global code executed and global variables initialized in perl?
2. If I write use package_name; in multiple packages, does the global code execute each time?
3. Are global variables defined this way thread safe?

Answer 1

Perl makes a complete copy of all code and variables for each thread. Communication between threads is via specially marked shared variables (which in fact are not shared - there is still a copy in each thread, but all the copies get updated). This is a significantly different threading model than many other languages have, so the thread-safety concerns are different - mostly centering around what happens when objects are copied to make a new thread and those objects have some form of resource to something outside the program (e.g. a database connection).

Your question about use isn't really related to threads, as far as I can tell? use does several things; one is loading the specified module and running any top-level code in it; this happens only once per module, not once per use statement.