There's no need to force anything. If you fixed test2 so it returned a list like it should, test wouldn't be evaluated in the same context as test2.

sub test {
   unless (defined wantarray) {
      print "test: wantarray is undefined\n";
   } elsif (wantarray) {
      print "test: wantarray is true\n";
   } else {
      print "test: wantarray is false\n";
   }
}

sub test2 {
   test();  # undefined
   return ...[ stuff to put in @t ]...;
}

my @t = test2();
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If you want the technical but ultimately useless details, see kennethk's post.