Most competent uses of globals such as $_, $/, etc, will involve localizing them, so this is not a problem unless you are coding poorly to begin with. In the specific case you've mentioned, grep auto-localizes $_, so there's nothing to worry about. 
   MeowChow                                   
               s aamecha.s a..a\u$&owag.print

In reply to (MeowChow) Re2: is $_ thread-safe? by MeowChow
in thread is $_ thread-safe? by princepawn

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