For a shared variable, each thread has a separate instance of it, each tied to a single, hidden variable that holds the value. When reading the tied variable, GET magic locks the hidden variable, copies out its value, then unlocks it. When writing the tied variable, PUT magic locks the hidden variable, replaces its value, then unlocks it. You don't need to do your own locking for threads::shared variables in the simple case.

You need to do locking if you have more than one (GET or PUT) operation and other iThreads might do other (GET or PUT) operations in between in a way that would cause problems. ||= does two operations so it is possible that extra locking needs to be done by the user (when dealing with shared variables). This likely means that even $x++ isn't iThreads safe on shared variables (in any case, I see no evidence of extra magic being implemented in the current threads::shared to make such safe -- probably easiest to do via overloading).