istg if that was the problem

index.json

@@ -2,6 +2,6 @@
   {
     "src": "src/startup.lua",
     "dest": "/startup.lua",
-    "hash": "07219cd9561b41ce1f39209958076c471b17855679c968b42767b0122423c782"
+    "hash": "af53be08c9478e5cb5fcb876ce580a2b45a69f191f60e466b88a4354dac1713a"
   }
 ]

src/startup.lua

@@ -1 +1,2 @@
 print("Hello, World!")
+

updateindex.nu

@@ -4,7 +4,7 @@
 open index.json
 | each {|entry| 
   {src: $entry.src, dest: $entry.dest, 
-  hash: (cat $entry.src | hash sha256)}
+  hash: (open $entry.src | hash sha256)}
 }
 | collect
 | save -f index.json