Fix multiple issues in t/op/magic.t

This commit fixes several critical issues in the magic variables test suite:

1. Fix RuntimeGlob cannot be cast to RuntimeCode crash (line 745)
   - Separated CODE and GLOB cases in RuntimeScalar.toStringRef()
   - Previously both cases were handled together causing incorrect type casting
   - Fixes fatal crash when converting glob to string reference

2. Initialize $^T magic variable with epoch time (test 98)
   - Added initialization of $^T to current Unix timestamp in GlobalContext
   - $^T represents script start time in seconds since epoch

3. Make ${^TAINT} read-only (test 127)
   - Use existing RuntimeScalarCache.scalarZero for ${^TAINT}
   - Leverages existing RuntimeScalarReadOnly infrastructure for immutability
   - Properly throws 'Modification of a read-only value' exception on write

4. Prevent auto-vivification of glob slots (test 85)
   - Modified RuntimeGlob.hashDerefGet() to check slot existence
   - *glob{HASH} and *glob{ARRAY} now return undef if slots don't exist
   - Prevents unwanted creation of hash/array slots on access

Test Results:
- Before: Multiple failures including fatal crash
- After: 158/208 tests passing (76% pass rate)
- Eliminated fatal crash, improved stability

Files Modified:
- RuntimeScalar.java: Fixed glob/code toStringRef() handling
- GlobalContext.java: Initialize $^T, use scalarZero for ${^TAINT}
- RuntimeGlob.java: Check existence before auto-vivifying slots

Author: fglock

src/main/java/org/perlonjava/runtime/GlobalContext.java

@@ -46,6 +46,7 @@ public static void initializeGlobals(CompilerOptions compilerOptions) {
         }
         GlobalVariable.getGlobalVariable("main::" + Character.toString('O' - 'A' + 1)).set(SystemUtils.getPerlOsName());    // initialize $^O
         GlobalVariable.getGlobalVariable("main::" + Character.toString('V' - 'A' + 1)).set(Configuration.getPerlVersionVString());    // initialize $^V
+        GlobalVariable.getGlobalVariable("main::" + Character.toString('T' - 'A' + 1)).set((int)(System.currentTimeMillis() / 1000));    // initialize $^T to epoch time
 
         // Initialize $^X - the name used to execute the current copy of Perl
         // PERLONJAVA_EXECUTABLE is set by the `jperl` or `jperl.bat` launcher
@@ -71,7 +72,7 @@ public static void initializeGlobals(CompilerOptions compilerOptions) {
         GlobalVariable.getGlobalVariable("main::?");
         GlobalVariable.getGlobalVariable("main::0").set(compilerOptions.fileName);
         GlobalVariable.getGlobalVariable(GLOBAL_PHASE).set(""); // ${^GLOBAL_PHASE}
-        GlobalVariable.getGlobalVariable(encodeSpecialVar("TAINT")); // ${^TAINT}
+        GlobalVariable.globalVariables.put(encodeSpecialVar("TAINT"), RuntimeScalarCache.scalarZero); // ${^TAINT} - read-only, always 0 (taint mode not implemented)
         GlobalVariable.getGlobalVariable("main::>");  // TODO
         GlobalVariable.getGlobalVariable("main::<");  // TODO
         GlobalVariable.getGlobalVariable("main::;").set("\034");  // initialize $; (SUBSEP) to \034

src/main/java/org/perlonjava/runtime/RuntimeGlob.java

@@ -186,8 +186,20 @@ public RuntimeScalar hashDerefGet(RuntimeScalar index) {
             case "CODE" -> GlobalVariable.getGlobalCodeRef(this.globName);
             case "IO" -> IO;
             case "SCALAR" -> GlobalVariable.getGlobalVariable(this.globName);
-            case "ARRAY" -> GlobalVariable.getGlobalArray(this.globName).createReference();
-            case "HASH" -> GlobalVariable.getGlobalHash(this.globName).createReference();
+            case "ARRAY" -> {
+                // Only return reference if array exists (has elements or was explicitly created)
+                if (GlobalVariable.existsGlobalArray(this.globName)) {
+                    yield GlobalVariable.getGlobalArray(this.globName).createReference();
+                }
+                yield new RuntimeScalar(); // Return undef if array doesn't exist
+            }
+            case "HASH" -> {
+                // Only return reference if hash exists (has elements or was explicitly created)
+                if (GlobalVariable.existsGlobalHash(this.globName)) {
+                    yield GlobalVariable.getGlobalHash(this.globName).createReference();
+                }
+                yield new RuntimeScalar(); // Return undef if hash doesn't exist
+            }
             case "FORMAT" -> GlobalVariable.getGlobalFormatRef(this.globName);
             default -> new RuntimeScalar();
         };

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

@@ -685,12 +685,18 @@ private String toStringLarge() {
     public String toStringRef() {
         String ref = switch (type) {
             case UNDEF -> "SCALAR(0x" + scalarUndef.hashCode() + ")";
-            case CODE, GLOB -> {
+            case CODE -> {
                 if (value == null) {
                     yield "CODE(0x" + scalarUndef.hashCode() + ")";
                 }
                 yield ((RuntimeCode) value).toStringRef();
             }
+            case GLOB -> {
+                if (value == null) {
+                    yield "GLOB(0x" + scalarUndef.hashCode() + ")";
+                }
+                yield ((RuntimeGlob) value).toStringRef();
+            }
             case VSTRING -> "VSTRING(0x" + value.hashCode() + ")";
             default -> "SCALAR(0x" + Integer.toHexString(value.hashCode()) + ")";
         };