The problem is that the entire while condition is evaluated every time around the loop, meaning that [@a] is evaluated every time around the loop, yielding a reference to a different anonymous array every time around the loop.

Because you're polling a fresh new array every time around the loop, each's "pointer" for the array is at the beginning every time.

This is the safe way to do it:

while (my ($i, $v) = each \@a) {
    say "$i: $v";
}
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... because you're using a reference to the same array every time around the loop.