That's not true. It's not parsed as if (0) { my $var = ''; }, and it does create a lexical $i (my's compile-time effect).

>perl -le "$::i=q{Pass }; for(1..3){my $i if 0; ++$i; print $::i, $i}"
Pass 1
Pass 2
Pass 3

>perl -le "$::i=q{Pass }; for(1..3){            ++$i; print $::i, $i}"
11
22
33
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What happens is that the clearing doesn't occur at the end of the scope (my's run-time effect). See Re: Lexical scope variable is not undef'ed.