How does that answers if $r->[1]@* is valid?

It doesn't, it answers only the question I understood  Why is the second arrow necessary in $r->[1]->@*?

I've no idea if  $r->[1]@* I usually don't run development builds of perl

From the documentation I would guess the answer is no, but the test suite might be more revealing https://metacpan.org/source/SHAY/perl-5.19.5/t/op/postfixderef.t ; short look, looks like answer is no