Localising does give you a local iterator. Try this code:

%a = ( A=>0, B=>1, C=>2 );
%b = %a;
print each %b, "\n";
{ local %b = %a; each %b; }
while(($k,$v)=each %b) { print $k, $v, "\n" }
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Then, try this:

%h = ( A=>0, B=>1, C=>2 );
print each %h, "\n";
() = %h;
print each %h, "\n";
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The problem with local %h = %h is not that the local copy shares the iterator, but that the original %h gets its iterator reset by being read. This is mentioned briefly in the documentation for the each function.