What are you expecting, and what did you get which doesn't meet with your expectation ?

FWIW, one wrinkle with threads is that the return context for the thread is set at threads->new time, not on the context of $thr->join(). Your code:

    $thr = threads->new(\&sub1, "THREAD1 :");
    ...
    @ReturnData = $thr->join; 
    ...
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should be:

    ($thr) = threads->new(\&sub1, "THREAD1 :");
    ...
    @ReturnData = $thr->join; 
    ...
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to provide the right context at the right time.

The fragment:

          {
            lock $foo;
            $foo=0;
          }
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is reasonably plausible. You're locking $foo for just long enough to give it a new value -- so there is no possibility of some other thread attempting to read or change $foo while it's in any intermediate state -- provided they too lock $foo before doing anything with it. (The lock is dropped at the end of the block it is contained in.)

I note that $foo is not initialised to anything.

I note that you:

    print " Final value :$foo\n";
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without locking it. But that's probably OK, because $foo is only changed by the same thread.

I wonder: you're not expecting $foo to work as some kind of semaphore, are you ?