wantarray, I thought, and your test would confirms it, depends on context, not scope.

But then, your do as block, with the code directly inside, like this:

my @arr2=do {wantarray
          ? print("wantarray!\n")
          : defined wantarray
            ? print("wantarray defined but false\n")
            : print("wantarray undefined!\n");
          wantarray
            ? qw( ciao a tutti )
            : defined wantarray && "howdy!";
        };
[download]

Is always undefined, both in scalar, void or array context, and the same apply to wantarray called directly, like this:

my $sc2=wantarray
          ? print("wantarray!\n") && qw(ciao a tutti)
          : defined wantarray
            ? print("wantarray defined but false\n") && "howdy!"
            : print("wantarray undefined!\n");
[download]

The way I got it working as expected were in a sub, like this:

sub wanttest {
  wantarray
    ? print("wantarray!\n")
    : defined wantarray
         ? print("wantarray defined but false\n")
         : print("wantarray undefined!\n");
  return wantarray
         ? qw( ciao a tutti )
         : defined wantarray && "howdy!";
}

wanttest();

my $sc=wanttest();
print "Returned $sc\n\n";

my @arr=wanttest();
print "Returned @arr \n\n";
[download]

Is wantarray only trustable inside subs or even there it fails to return as expected anytime?

I was expecting thet the context made the trick, but after see it fail when called directly, I wasn't expecting that the do block didn't work as the do EXPR (do 'filename.pl').