Make make-block.pl's output more flexible

make-block.pl

@@ -53,7 +53,7 @@ sub add_host_file {
 }
 
 MAIN: {
-    my $out = 'block.list';
+    my $out;
     my $block_ip = '0.0.0.0';
     my $workdir = &dirname($0);
 
@@ -77,8 +77,11 @@ MAIN: {
     }
 
     my $written = 0;
-    open my $fho, '>', $out
-        or die "Failed to open file $out for writing: $!";
+    my $fho = \*STDOUT;
+    if (defined $out && length $out) {
+        open $fho, '>', $out
+            or die "Failed to open file $out for writing: $!";
+    }
     print $fho map {
         ++$written;
         "$block_ip $_\n"