Please, why the statement my $a, $x, $y, $z = foo() is in void context? I don't get it.
If you are unsure about Perl's concept of context, see e.g. the Context tutorial, the section "Context" in Chapter 2 of the Camel, or the section "Context" in Modern Perl.
In the example I showed, perl -e 'my $a, $x, $y, $z = foo()', the statement is in void context because it is the only statement in the program and its return value is not being used anywhere.
In my @foo = ( my $a, $x, $y, $z = foo() ); the statement is in list context because it is being assigned to an array, and in my $bar = ( my $a, $x, $y, $z = foo() ); it's in scalar context. For the last statement in a sub, the context of the caller is used, which is why in the above example, my @x = bar(); the statement my $a, $x, $y, $z = foo(); is in list context and in my $r = bar(); it's in scalar context. If you were to write my $a, $x, $y, $z = foo(); 1;, that would also force void context on the statement.