This gotcha is explained in the standard documentation for the conditional operator in perlop:

       Because this operator produces an assignable result, using
       assignments without parentheses will get you in trouble.
       For example, this:
 
           $a % 2 ? $a += 10 : $a += 2
 
       Really means this:
 
           (($a % 2) ? ($a += 10) : $a) += 2
 
       Rather than this:
 
           ($a % 2) ? ($a += 10) : ($a += 2)
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