I see.

So, with global variables, like in

$a = 2; sub f { print $a; } { local $a = 5; f; }'
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or

$a = 2; sub f { print $a; } for $a (5) { f; }
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we get 5 as the result, as perl localizes $a dynamically.

However, if you put my before $a, perl can not localize the variable dynamically. In the first case, local raises an error, as Larry knows Perl can't localize a my, so he explicitly denied it. In the second case, however, if you put my before $a in the beginning, for will not carp (that would be a too bad restriction), but instead localizes $a in the wrong way.