Read question 2 more carefully.

Question 1 has to do with $a passing "use strict" for the reason you offer.

But question 2 has to do with warnings on math with undef values. Although $a passes strict in this puzzle, it is still undef at the point we first encounter it.

Just about any kind of math on such a value will not abort (as happens with the use strict violation referred to in Q. 1), but it normally does produce a warning (if warnings are turned on) e.g.:

     $a = $a + 1;    # warns but executes anyway
     print $a;       # 1
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In this case, the warning is: "Use of uninitialized value in addition (+) at testprog.pl line xx.". So question 2 is: Why is such a warning not produced in the snippet meryln refers to.