In

while ( ($key, $value) = each %hash ) {
  ...
}
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The operator the while is evaluating is the list assignment operator. ($key, $value) and each %hash are the operands to the list assignment operator. It's as if you have list_assign(list(lexical_sv('key'), lexical_sv('value')), each(lexical_hv('hash'))). As you can see, the list assignment operator is what is in scalar context, not ($key, $value). To see what the list assignment operator returns in scalar context, see Mini-Tutorial: Scalar vs List Assignment Operator, but it's easy to figure out.

>perl -E"say scalar(  ($x,$y)=()  )"
0

>perl -E"say scalar(  ($x,$y)=(undef)  )"
1

>perl -E"say scalar(  ($x,$y)=(undef,undef)  )"
2

>perl -E"say scalar(  ($x,$y)=(undef,undef,undef)  )"
3
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