Actually, the first 3 examples work exactly how the simple model I offered in my previous posts would expect them to.

Perhaps, but they don't all behave according to the documentation which say each of the loop pairs I provided should behave the same. Thus the deviation from the documentation is Perl being "confused".

However, I must concur that it's very predictable confusion.

But I can see why "Don't do it" is the better answer.

There are way less obscure ways to structure the code this optimisation enables.

Stack and queues are by far the most common usage of modifying an array being processed, and both are quite clear and concise without foreach.

while (@todo) {
   my $item = pop(@todo);
   ...
   push @todo, ...;
   ...   
}
[download]

while (@todo) {
   my $item = shift(@todo);
   ...
   push @todo, ...;
   ...   
}
[download]