Well ... maybe show us an example from another language w/o "this limitation" to learn from? :)

use feature qw/say/;
my @bb = 1..10; 
while(my ($ind, $val) = each @bb){
    say "ind is $ind";
           if($ind >5){
                while(my ($inner_ind, $inner_val) = each @bb){
                   say "inner ind is $inner_ind, inner val is $inner_v
+al"
        }
    }
}

# will endless loop since while share one iterator

while(my ($ind, $val) = each @bb){
    say "ind is $ind";
}
# this will quit loop successfully.
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I don't understand why every array has to have only one iterator, please enlighten me. and guess each/keys doesn't work on slice would be relative to this reason?

> > maybe show us an example from another language w/o "this limitation" to learn from? :)

these are not examples from another language demonstrating no "limitations"... :)

anyway in Perl 5.36 you can do this

use v5.36;
use warnings;
use experimental qw/for_list/;

my %hash;
@hash{"a".."e"} = 1..5;

for my ($key, $value) ( %hash{"c","a","e"} ) {
    say "$key -> $value";
}
[download]

$ perl  /d/perl/pm/iterate_hash_slice.pl
c -> 3
a -> 1
e -> 5
[download]

Cheers Rolf
_{(addicted to the 𐍀𐌴𐍂𐌻 Programming Language :)
Wikisyntax for the Monastery}

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.)