as this demonstates, wantarray is defined in both scalar and list context (which is why your testc1 fails), but true only in list context.

sub testc {
    if (defined wantarray() and not wantarray()) {
        print "scalar\n";
    } elsif (defined wantarray() and wantarray()) {
        print "array\n";
    } elsif (not defined wantarray()) {
        print "void\n";
    }
}
my $x = testc();
my @x = testc();
testc();
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