1. It enables strict 'refs', which prevents strings from being accidentally used as references. None of the examples in the book used references at all, so there was no reason to use strict 'refs'.
   
2. It enables strict 'vars', which prevents global variables from being used without being declared; typically, one declares variables local with the my declaration. While this is good practice in general, the example programs were all very small---less than twenty lines each. In such small programs, there is no practical difference between a global variable and one that has been declared with my. No benefit would have accrued from requiring the use of my declarations of every variable.
3. It enables strict 'subs', which is of very limited value even at the best of times. strict 'subs' forbids unquoted strings, because such strings ('barewords') can cause long-term maintenance problems. If you have code like
   

        if ($x eq carrots) {
          ...
        }
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the carrots is taken as a literal string. But if someone later adds a carrots() function to the program, the meaning of this line might change suddenly and unexpectedly, to call carrots() and compare $x with the returned value. This is not too likely, except perhaps in very large and long-lived programs, which is why strict 'subs' is of such limited value. In 20-line book examples, it is of no value whatsoever.