With $length = (4,5,6) the thing that gets evaluated in scalar context is (4,5,6), which a list, and so indeed, $length here will be assigned the value '6'. But if you apply scalar context to something else, the result is not always the last element. For exemple in $length = @array the right side is an array, not a list (the two notions are close but not identical in perl, you can see an array as a list encapsulated in an object that changes its behavior), and you get the length. And, another case, reverse in list context will return a list of elements, but in scalar context does not just return the last elements, but the concatenated reverse string.

The reason that the while(($key, $val) = each %hash) works is because the expression that gets evaluated in scalar context is not a list, but an assignment (it's not just data, it's an action), and list assignments, unlike simple lists eval to the size of the list. You can see ($key, $val) = each %hash as a simpler way to write AssignTo(key => \$key, val => $val, from => \%hash) (this does not work), where AssignTo is a fonction that returns the number of elements 'removed' from the hash. If indeed, the while tested the last value, and not the number of elements, a hash like (1 => 0, 2 => 0) would fail to be read by the loop, because $val would be 0, which is false.

The difference between just a list and a list assignment is used in this kind of code: my $numberOfMatches = () = /regex/g; Where the right part is evaluated first (so the matches of the regex are copied to (), ie thrown away), and then my $numberOfMatches = LIST_ASSIGNMENT is evaluated, where the value of LIST_ASSIGNMENT is the number of elements that where redirected towards ().

Edit: I feel like I wrote too much an wasn't really clear I hope I didn't confuse you even more ...

 scalar (3 .. 4) in scalar context is the flip flop operator, not a range and its false hence empty string.

 scalar 3 .. 4 is just  (scalar 3) .. 4

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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In my opinion, after "($key, $value)" are assigned to "()" and list becomes "(undef, undef)", scalar context takes only the last element of the list, that means 2nd "undef". Am I wrong?

Compare:

$ perl -E 'say scalar ( ($a, $b) = () )'
0
$ perl -E 'say scalar ( ($a, $b) = (undef, undef) )'
2
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Thanks for all of you!
That's a bit difficult, but I'll try to understand, memorize.