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.
[download]

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?