No, it's not. And if you turn warnings on, Perl will point out to you that you may make a mistake. Note that in many cases, it's actually harmless.

In the early years of Perl5, the for my $var syntax was illegal. If you wanted a lexical variable for your iterator, you'd write:

my $i;
for $i (LIST) { ... }
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It would not be uncommon to have several my $i;'s in the same block.

But that's harmless.