I don't understand why every array has to have only one iterator

First of all, it's one more than needed.

for my $ind ( 0 .. $#bb ) {
   say "ind is $ind";
}
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And to answer your question, it's the maximum you can have. If you want multiple iterators, they necessarily have to be external to the array.

For example, in C#, you have to create an external iterator.

using ( var enumerator = list.GetEnumerator() ) {
   while ( enumerator.MoveNext() ) {
      var item = emumerator.current;
      // ...
   }
}
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Nothing stops you from doing the same in Perl.

Now, C# does have syntactical sugar that makes this cleaner.

for ( var item in list ) {
   // ...
}
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Again, nothing stops you from doing the same in Perl.

If no one's done it, I guess it's not that useful. (I'm not saying noone's done it. I have no idea.)