strict is smart enough to detect variables that are not properly declared, but not necessarily to detect variables that are logically misused. In this case, declaring a 'my' variable inside a for loop is perfectly legal (in fact, I do it all the time without thinking much about it), e.g.

foreach my $foo (@ary) {
  . . .
}
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But for a running summation, of course it's not what you want. So strict doesn't see a problem; although it's misused, the variable is properly declared.