diff --git a/src/main/java/org/perlonjava/EmitterVisitor.java b/src/main/java/org/perlonjava/EmitterVisitor.java
index e08c3c42..d3dc7bb5 100644
--- a/src/main/java/org/perlonjava/EmitterVisitor.java
+++ b/src/main/java/org/perlonjava/EmitterVisitor.java
@@ -495,6 +495,10 @@ private void handleUnaryBuiltin(UnaryOperatorNode node, String operator) throws
             // Unary operator with optional arguments, called without arguments
             // example: undef()
             ctx.mv.visitMethodInsn(Opcodes.INVOKESTATIC, "org/perlonjava/RuntimeScalar", operator, "()Lorg/perlonjava/RuntimeScalar;", false);
+        } else if (operator.equals("undef")) {
+            operator = "undefine";
+            node.operand.accept(this.with(RuntimeContextType.RUNTIME));
+            ctx.mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "org/perlonjava/RuntimeList", operator, "()Lorg/perlonjava/RuntimeList;", false);
         } else {
             node.operand.accept(this.with(RuntimeContextType.SCALAR));
             ctx.mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "org/perlonjava/RuntimeScalar", operator, "()Lorg/perlonjava/RuntimeScalar;", false);
diff --git a/src/main/java/org/perlonjava/Parser.java b/src/main/java/org/perlonjava/Parser.java
index 55285252..f5cce0b1 100644
--- a/src/main/java/org/perlonjava/Parser.java
+++ b/src/main/java/org/perlonjava/Parser.java
@@ -319,9 +319,6 @@ private Node parsePrimary() {
                     return new StringNode(token.text, tokenIndex);
                 }
                 switch (token.text) {
-                    case "undef":
-                        // Handle 'undef' keyword as a unary operator with no operand
-                        return new UnaryOperatorNode("undef", null, tokenIndex);
                     case "not":
                         // Handle 'not' keyword as a unary operator with an operand
                         operand = parseExpression(getPrecedence(token.text) + 1);
@@ -329,16 +326,22 @@ private Node parsePrimary() {
                     case "abs":
                     case "log":
                     case "rand":
+                    case "undef":
                         String text = token.text;
                         operand = parseZeroOrOneList(0);
                         if (((ListNode) operand).elements.isEmpty()) {
-                            if (text.equals("rand")) {
-                                // create "1"
-                                operand = new NumberNode("1", tokenIndex);
-                            } else {
-                                // create `$_` variable
-                                operand = new UnaryOperatorNode(
+                            switch (text) {
+                                case "undef":
+                                    break;  // leave it empty
+                                case "rand":
+                                    // create "1"
+                                    operand = new NumberNode("1", tokenIndex);
+                                    break;
+                                default:
+                                    // create `$_` variable
+                                    operand = new UnaryOperatorNode(
                                         "$", new IdentifierNode("_", tokenIndex), tokenIndex);
+                                    break;
                             }
                         }
                         return new UnaryOperatorNode(text, operand, tokenIndex);
diff --git a/src/main/java/org/perlonjava/RuntimeArray.java b/src/main/java/org/perlonjava/RuntimeArray.java
index 33d32d0e..7c86fa8e 100644
--- a/src/main/java/org/perlonjava/RuntimeArray.java
+++ b/src/main/java/org/perlonjava/RuntimeArray.java
@@ -276,6 +276,11 @@ public boolean getBooleanRef() {
       return true;
     }
 
+    public RuntimeArray undefine() {
+        this.elements.clear();
+        return this;
+    }
+
     // Convert the array to a string (for debugging purposes)
     @Override
     public String toString() {
diff --git a/src/main/java/org/perlonjava/RuntimeHash.java b/src/main/java/org/perlonjava/RuntimeHash.java
index 8f070c52..05ef2612 100644
--- a/src/main/java/org/perlonjava/RuntimeHash.java
+++ b/src/main/java/org/perlonjava/RuntimeHash.java
@@ -233,6 +233,11 @@ public boolean getBooleanRef() {
       return true;
     }
 
+    public RuntimeHash undefine() {
+        this.elements.clear();
+        return this;
+    }
+
     // Convert the hash to a string (for debugging purposes)
     @Override
     public String toString() {
diff --git a/src/main/java/org/perlonjava/RuntimeList.java b/src/main/java/org/perlonjava/RuntimeList.java
index 78b52656..273ba302 100644
--- a/src/main/java/org/perlonjava/RuntimeList.java
+++ b/src/main/java/org/perlonjava/RuntimeList.java
@@ -148,6 +148,20 @@ public Iterator<RuntimeScalar> iterator() {
 
     // Operators
 
+    // undefine the elements of the list
+    public RuntimeList undefine() {
+        for (RuntimeBaseEntity elem : elements) {
+            if (elem instanceof RuntimeScalar) {
+                ((RuntimeScalar) elem).undefine();
+            } else if (elem instanceof RuntimeArray) {
+                ((RuntimeArray) elem).undefine();
+            } else if (elem instanceof RuntimeHash) {
+                ((RuntimeHash) elem).undefine();
+            }
+        }
+        return this;
+    }
+
     public RuntimeScalar print() {
       StringBuilder sb = new StringBuilder();
         for (RuntimeBaseEntity element : elements) {
diff --git a/src/main/java/org/perlonjava/RuntimeScalar.java b/src/main/java/org/perlonjava/RuntimeScalar.java
index d59595d3..3dd82410 100644
--- a/src/main/java/org/perlonjava/RuntimeScalar.java
+++ b/src/main/java/org/perlonjava/RuntimeScalar.java
@@ -449,6 +449,11 @@ public static RuntimeScalar undef() {
         return new RuntimeScalar();
     }
 
+    public RuntimeScalar undefine() {
+        this.type = RuntimeScalarType.UNDEF;
+        return this;
+    }
+
     public RuntimeScalar stringConcat(RuntimeScalar b) {
         return new RuntimeScalar(this + b.toString());
     }