Use wantarray:

sub test {
   print(defined(wantarray)?1:0, " ");
   print(wantarray?1:0, "\n");
}

test();       # 0 0
$a = test();  # 1 0
@a = test();  # 1 1
[download]

sub function {
   my $rv = "Hello World";

   if (defined wantarray) {
      return $rv;
   } else {
      print("$rv\n");
      return;
   }
}

function();              # Hello World

my $greet = function();
print("[$greet]\n");     # [Hello World]
[download]