In my opinion, you're abusing the "given/when" construct. Yes, it works, because you have a boolean expression in the when() clauses. But you could put anything in the given part, and it would work just the same.

The point of "given/when", is that the value in the given clause is assigned to $_, and then you can use invisible default $_ in all the when() tests. From "perldoc perlsyn":

   given($_) {
      when (/^abc/) { $abc = 1; }
      when (/^def/) { $def = 1; }
      when (/^xyz/) { $xyz = 1; }
      default { $nothing = 1; }
   }
# and similarly
   given($foo) {
      when (undef) {
         say '$foo is undefined';
      }
      when ("foo") {
         say '$foo is the string "foo"';
      }
      when ([1,3,5,7,9]) {
         say '$foo is an odd digit';
         continue; # Fall through
      }
      when ($_ < 100) {
         say '$foo is numerically less than 100';
      }
      when (\&complicated_check) {
         say 'a complicated check for $foo is true';
      }
      default {
         die q(I don't know what to do with $foo);
      }
   }

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As far as I can see, you're using a given/when block to produce an if/elsif block with an extra level of indentation.

If you modify your foo/bar/baz example so that each print block has a new statement after it: pritn "\$_ is $_.\n";. That will print out what the value of $_ is, no matter which path you take. You will see that $_ is 'baz'. Your ($foo, $bar, $baz) evaluates the sequence of expressions and uses the last one as the chosen value. The earlier ones are evaluated only for their side-effect. Of course, $foo does not have a side-effect, but it could equally be $foo++, or $foo->(). That's why you're getting the message about "useless use of a private variable", because only the last one gets assigned to $_.

As Occam said: Entia non sunt multiplicanda praeter necessitatem.

In reply to Re^3: Text Analysis: given(){when(){}} block throws a 'useless use of private variable...etc' error by TomDLux
in thread Text Analysis: given(){when(){}} block throws a 'useless use of private variable...etc' error by biohisham

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