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

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