No, the loop won't exit even if [ undef ] or [ 0 ] is returned. List assignment in scalar context returns the number of elements to which its RHS evaluates, meaning the number of elements in the array.

You'd be right that

while( my $next = $iter->() ) {
    my @next = @$next;
    ...
}
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is not equivalent to

while( my @next = @{ $iter->() } ) {
    ...
}
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But that's not the situation here.

The point was that

while( my $next = $iter->() ) {
    ($next) = @$next;
    ...
}
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while( my $next = @{ $iter->() } ) {
    ...
}
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which is just an expensive version of

while( my ($next) = $iter->() ) {
    ...
}
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The point was that 
while( my $next = $iter->() ) {
    ($next) = @$next;
    ...
}

is a complicated version of 
while( my $next = @{ $iter->() } ) {
    ...
}
[download]

Not so. In the latter case when the iterator returns undef to terminate iteration, you will attempt to dereference that undef causing an error.

In the former, the loop terminates and the dereference is never reached.

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

No, the loop won't exit even if [ undef ] or [ 0 ] is returned.

It isn't supposed to. The loop exists when you return undef to stop the loop. Read carefully :) Dumb, non-question: How to return 'nothing'.