perl locks not just variable but the whole shared space, is it correct?

All shared variables are kept in a separate interpreter, with stub copies of the variable in each thread, and a tieing mechanism used to access the vars. So when you do something like $shared_var = 1, your thread notes that $shared_var is tied, an calls the STORE method (which happens to be implemented in XS for efficiency). This method sets a global lock and copies the new value to the 'real' variable.

Dave.