Patches to perl invaders

diff --git a/bin/perl_invaders.pl b/bin/perl_invaders.pl
index 6776d39..dc90718 100644
--- a/bin/perl_invaders.pl
+++ b/bin/perl_invaders.pl
@@ -1,3 +1,4 @@
+#!perl -w
 use 5.010; use strict; use warnings;
 use SDL::App;
 use SDL::Event;
@@ -83,7 +84,7 @@ while (1) {
     $app->background->blit($onion->rect,$app->window,$onion->rect);
     $onion->rect->x($onion->rect->x + $dir_x);
     $onion->rect->y($onion->rect->y + $dir_y);
-    $onion->surface->blit( undef, $app->window, $onion->rect );
+    $onion->surface->blit( SDL::NULL, $app->window, $onion->rect );

     $app->window->sync;
 }
diff --git a/lib/Game/PerlInvaders/App.pm b/lib/Game/PerlInvaders/App.pm
index 3bbaf23..3b0ad8c 100644
--- a/lib/Game/PerlInvaders/App.pm
+++ b/lib/Game/PerlInvaders/App.pm
@@ -19,7 +19,7 @@ sub BUILD {
     -depth  => 16,

     ));
-    $self->background->blit( undef, $self->window, undef );
+    $self->background->blit( SDL::NULL, $self->window, SDL::NULL );

 }

@@ -53,7 +53,7 @@ sub draw_sprite {
     $sprite->rect->x($sprite->position_x);
     $sprite->rect->y($sprite->position_y);

-    $sprite->surface->blit( undef, $self->window, $sprite->rect );
+    $sprite->surface->blit( SDL::NULL, $self->window, $sprite->rect );
 }

 sub move_like_a_space_invader {