Fix interpreter strict vars check to allow use-vars-declared globals

src/main/java/org/perlonjava/backend/bytecode/BytecodeCompiler.java

@@ -396,8 +396,8 @@ boolean shouldBlockGlobalUnderStrictVars(String varName) {
             return false;
         }
 
-        // Allow regex capture variables ($1, $2, etc.)
-        if (ScalarUtils.isInteger(bareVarName)) {
+        // Allow regex capture variables ($1, $2, etc.) but not leading-zero variants ($01, $02)
+        if (ScalarUtils.isInteger(bareVarName) && !bareVarName.startsWith("0")) {
             return false;
         }
 
@@ -423,6 +423,39 @@ boolean shouldBlockGlobalUnderStrictVars(String varName) {
             return false;
         }
 
+        // Allow variables that already exist in the global registry
+        // (e.g., created by `use vars` at parse time)
+        // This mirrors the allowIfAlreadyExists logic in EmitVariable.java
+        String normalizedName = NameNormalizer.normalizeVariableName(bareVarName, getCurrentPackage());
+        boolean allowIfAlreadyExists = false;
+        if (sigil.equals("$") && GlobalVariable.existsGlobalVariable(normalizedName)) {
+            allowIfAlreadyExists = true;
+        }
+        if (sigil.equals("@") && GlobalVariable.existsGlobalArray(normalizedName)) {
+            allowIfAlreadyExists = true;
+        }
+        if (sigil.equals("%") && !normalizedName.endsWith("::") && GlobalVariable.existsGlobalHash(normalizedName)) {
+            allowIfAlreadyExists = true;
+        }
+
+        // Perl's strict 'vars' requires declaration for unqualified single-letter globals
+        // even if they were previously created under 'no strict'.
+        // This mirrors EmitVariable.java lines 349-359.
+        boolean isSpecialSortVar = sigil.equals("$")
+                && (bareVarName.equals("a") || bareVarName.equals("b"));
+        if (sigil.equals("$")
+                && bareVarName != null
+                && bareVarName.length() == 1
+                && Character.isLetter(bareVarName.charAt(0))
+                && !bareVarName.contains("::")
+                && !isSpecialSortVar) {
+            allowIfAlreadyExists = false;
+        }
+
+        if (allowIfAlreadyExists) {
+            return false;
+        }
+
         // BLOCK: Unqualified variable under strict vars
         return true;
     }

src/main/java/org/perlonjava/backend/jvm/EmitVariable.java

@@ -361,7 +361,7 @@ static void handleVariableOperator(EmitterVisitor emitterVisitor, OperatorNode n
 
                 // Compute createIfNotExists flag - determines if variable can be auto-vivified
                 boolean createIfNotExists = name.contains("::")  // Fully qualified: $Package::var
-                        || ScalarUtils.isInteger(name)           // Regex capture: $1, $2, etc.
+                        || (ScalarUtils.isInteger(name) && !name.startsWith("0"))  // Regex capture: $1, $2, etc.
                         || isSpecialSortVar                      // Sort variables: $a, $b
                         || isBuiltinSpecialLengthOneVar(sigil, name) // $%, $-, $[, $}, etc.
                         || isBuiltinSpecialScalarVar(sigil, name) // ${^GLOBAL_PHASE}, $ARGV, $ENV, etc.

src/main/java/org/perlonjava/runtime/runtimetypes/RuntimeIO.java

@@ -912,7 +912,7 @@ private Set<StandardOpenOption> convertMode(String mode) {
 
         Set<StandardOpenOption> options = MODE_OPTIONS.get(mode);
         if (options == null) {
-            throw new PerlCompilerException("Unsupported file mode: " + mode);
+            throw new PerlCompilerException("Unknown open() mode '" + mode + "'");
         }
         return new HashSet<>(options);
     }

src/main/java/org/perlonjava/runtime/runtimetypes/RuntimeList.java

@@ -329,7 +329,8 @@ public RuntimeScalar scalar() {
             return scalarUndef; // Return undefined if empty
         }
         // XXX expand the last element
-        return elements.getLast().scalar();
+        RuntimeBase last = elements.getLast();
+        return (last == null) ? scalarUndef : last.scalar();
     }
 
     /**
@@ -464,8 +465,8 @@ public RuntimeArray setFromList(RuntimeList value) {
                     rhsIndex++;
                 }
             } else if (elem instanceof RuntimeScalar runtimeScalar) {
-                RuntimeScalar assigned = (rhsIndex < rhsSize) ? rhsElements.get(rhsIndex++) : new RuntimeScalar();
-                runtimeScalar.set(assigned);
+                RuntimeScalar assigned = (rhsIndex < rhsSize) ? rhsElements.get(rhsIndex++) : null;
+                runtimeScalar.set(assigned != null ? assigned : new RuntimeScalar());
                 result.elements.add(runtimeScalar);  // Add reference to the variable itself
             } else if (elem instanceof RuntimeArray runtimeArray) {
                 List<RuntimeScalar> remaining = (rhsIndex < rhsSize)

src/main/java/org/perlonjava/runtime/runtimetypes/RuntimeScalar.java

@@ -640,6 +640,11 @@ public RuntimeScalar addToScalar(RuntimeScalar scalar) {
 
     // Setters
     public RuntimeScalar set(RuntimeScalar value) {
+        if (value == null) {
+            this.type = RuntimeScalarType.UNDEF;
+            this.value = null;
+            return this;
+        }
         if (value.type == TIED_SCALAR) {
             return set(value.tiedFetch());
         }