You can force VOID context by using Gives. You can also get greater control of wantarray by using Want. Here's what I did:

#!/usr/bin/perl

use strict;
use warnings;
use Gives;
use Want 'want';

sub test : Gives( @Contexts ) {
   if (defined want('VOID')) {
       print "test: wantarray is undefined\n";
   } 
   elsif (want('SCALAR')) {
       print "test: wantarray is true\n";
   } 
   else {
       print "test: wantarray is false\n";
   }
}

sub test2 {
   test();
}
print test2;
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