Something like this is fairly commonly seen:

while( my ( $z, $y, $x, $w ) = @{ $iter->next // [] } ) {
   ...;
}
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When next returns undef, you end up assigning an empty array to ( $z, $y, $x, $w ). List assignment evaluates to the size of the array on the right hand side, which is zero, thus false, and exits the loop.

In place of // you could use || or or.