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+---
+name: debug-perlonjava
+description: Debug and fix test failures and regressions in PerlOnJava
+argument-hint: "[test-name, error message, or Perl construct]"
+triggers:
+  - user
+  - model
+---
+
+# Debugging PerlOnJava
+
+You are debugging failures in PerlOnJava, a Perl-to-JVM compiler with a bytecode interpreter fallback. This skill covers debugging workflows for test failures, regressions, and parity issues between backends.
+
+## Project Layout
+
+- **PerlOnJava source**: `src/main/java/org/perlonjava/` (compiler, bytecode interpreter, runtime)
+- **Unit tests**: `src/test/resources/unit/*.t` (156 tests, run via `mvn test`)
+- **Perl5 core tests**: `perl5_t/t/` (Perl 5 compatibility suite)
+- **Fat JAR**: `target/perlonjava-3.0.0.jar`
+- **Launcher script**: `./jperl`
+
+## Building
+
+```bash
+mvn package -q -DskipTests    # Build JAR (required after any Java change)
+mvn test                       # Run unit tests (156 tests, must all pass)
+```
+
+## Running Tests
+
+### Single Perl5 core test
+```bash
+cd perl5_t/t
+../../jperl op/bop.t
+```
+
+### With environment variables (for specific tests)
+```bash
+# For re/pat.t and similar regex tests
+JPERL_UNIMPLEMENTED=1 JPERL_OPTS=-Xss256m PERL_SKIP_BIG_MEM_TESTS=1 ./jperl perl5_t/t/re/pat.t
+
+# For op/sprintf2.t
+JPERL_UNIMPLEMENTED=1 ./jperl perl5_t/t/op/sprintf2.t
+```
+
+### Test runner (parallel, with summary)
+```bash
+perl dev/tools/perl_test_runner.pl perl5_t/t/op
+perl dev/tools/perl_test_runner.pl --jobs 8 --timeout 60 perl5_t/t
+```
+
+### Interpreter mode
+```bash
+./jperl --interpreter script.pl
+./jperl --interpreter -e 'print "hello\n"'
+JPERL_INTERPRETER=1 ./jperl script.pl   # Global (affects require/do/eval)
+```
+
+## Comparing Outputs
+
+### PerlOnJava vs System Perl
+```bash
+# System Perl
+perl -e 'my @a = (1,2,3); print "@a\n"'
+
+# PerlOnJava
+./jperl -e 'my @a = (1,2,3); print "@a\n"'
+```
+
+### JVM backend vs Interpreter backend
+```bash
+./jperl -e 'code'                      # JVM backend
+JPERL_INTERPRETER=1 ./jperl -e 'code'  # Interpreter backend
+```
+
+### Disassembly comparison
+```bash
+./jperl --disassemble -e 'code'                      # JVM bytecode
+./jperl --disassemble --interpreter -e 'code'        # Interpreter bytecode
+```
+
+## Environment Variables
+
+### Compiler/Interpreter Control
+| Variable | Effect |
+|----------|--------|
+| `JPERL_INTERPRETER=1` | Force interpreter mode globally (require/do/eval) |
+| `JPERL_DISABLE_INTERPRETER_FALLBACK=1` | Disable bytecode interpreter fallback for large subs |
+| `JPERL_SHOW_FALLBACK=1` | Print message when a sub falls back to interpreter |
+| `JPERL_EVAL_NO_INTERPRETER=1` | Disable interpreter for `eval STRING` |
+| `JPERL_OPTS="-Xss256m"` | Pass JVM options (e.g., stack size) |
+
+### Debugging/Tracing
+| Variable | Effect |
+|----------|--------|
+| `JPERL_DISASSEMBLE=1` | Disassemble generated bytecode |
+| `JPERL_ASM_DEBUG=1` | Print JVM bytecode when ASM frame computation crashes |
+| `JPERL_EVAL_VERBOSE=1` | Verbose error reporting for eval compilation |
+| `JPERL_EVAL_TRACE=1` | Trace eval STRING execution path |
+| `JPERL_IO_DEBUG=1` | Trace file handle open/dup/write operations |
+| `JPERL_REQUIRE_DEBUG=1` | Trace `require`/`use` module loading |
+
+### Perl-level
+| Variable | Effect |
+|----------|--------|
+| `JPERL_UNIMPLEMENTED=1` | Allow unimplemented features (skip instead of die) |
+| `PERL_SKIP_BIG_MEM_TESTS=1` | Skip memory-intensive tests |
+
+## Debugging Workflow
+
+### 1. Identify the regression
+```bash
+# Compare branch vs master
+git checkout master && mvn package -q -DskipTests
+./jperl -e 'failing code'
+
+git checkout branch && mvn package -q -DskipTests
+./jperl -e 'failing code'
+```
+
+### 2. Bisect to find the bad commit
+```bash
+git log master..branch --oneline
+git checkout <commit> && mvn package -q -DskipTests && ./jperl -e 'test'
+```
+
+### 3. Create minimal reproducer
+Reduce the failing test to the smallest code that demonstrates the bug:
+```bash
+./jperl -e 'my $x = 58; eval q{($x) .= "z"}; print "x=$x\n"'
+```
+
+### 4. Compare with system Perl
+```bash
+perl -e 'same code'
+```
+
+### 5. Use disassembly to understand
+```bash
+./jperl --disassemble -e 'minimal code' 2>&1 | grep -i "relevant"
+```
+
+### 6. Add debug prints (if needed)
+In Java source, add:
+```java
+System.err.println("DEBUG: var=" + var);
+```
+Then rebuild with `mvn package -q -DskipTests`.
+
+### 7. Fix and verify
+```bash
+# After fixing
+mvn package -q -DskipTests
+./jperl -e 'test code'        # Verify fix
+mvn test                       # No regressions in unit tests
+```
+
+## Architecture: Two Backends
+
+```
+Source → Lexer → Parser → AST ─┬─→ JVM Compiler → JVM bytecode (default)
+                                └─→ BytecodeCompiler → InterpretedCode → BytecodeInterpreter
+```
+
+Both backends share the parser (same AST) and runtime (same operators, same RuntimeScalar/Array/Hash).
+
+## Key Source Files
+
+| Area | File | Notes |
+|------|------|-------|
+| **Bytecode Compiler** | `backend/bytecode/BytecodeCompiler.java` | AST → interpreter bytecode |
+| **Bytecode Interpreter** | `backend/bytecode/BytecodeInterpreter.java` | Main dispatch loop |
+| **Assignment (interp)** | `backend/bytecode/CompileAssignment.java` | Assignment compilation |
+| **Binary ops (interp)** | `backend/bytecode/CompileBinaryOperator.java` | Binary operator compilation |
+| **Unary ops (interp)** | `backend/bytecode/CompileOperator.java` | Unary operator compilation |
+| **Opcodes** | `backend/bytecode/Opcodes.java` | Opcode constants |
+| **JVM Compiler** | `backend/jvm/EmitterMethodCreator.java` | AST → JVM bytecode |
+| **JVM Subroutine** | `backend/jvm/EmitSubroutine.java` | Sub compilation (JVM) |
+| **JVM Binary ops** | `backend/jvm/EmitBinaryOperator.java` | Binary ops (JVM) |
+| **Compilation router** | `app/scriptengine/PerlLanguageProvider.java` | Picks backend |
+| **Runtime scalar** | `runtime/runtimetypes/RuntimeScalar.java` | Scalar values |
+| **Runtime array** | `runtime/runtimetypes/RuntimeArray.java` | Array values |
+| **Runtime hash** | `runtime/runtimetypes/RuntimeHash.java` | Hash values |
+| **Math operators** | `runtime/operators/MathOperators.java` | +, -, *, /, etc. |
+| **String operators** | `runtime/operators/StringOperators.java` | ., x, etc. |
+| **Bitwise operators** | `runtime/operators/BitwiseOperators.java` | &, |, ^, etc. |
+| **Regex runtime** | `runtime/regex/RuntimeRegex.java` | Regex matching |
+| **Regex preprocessor** | `runtime/regex/RegexPreprocessor.java` | Perl→Java regex |
+
+All paths relative to `src/main/java/org/perlonjava/`.
+
+## Common Bug Patterns
+
+### 1. Context not propagated correctly
+**Symptom**: Operation returns wrong type (list vs scalar).
+**Pattern**: Code uses `node.accept(this)` instead of `compileNode(node, -1, RuntimeContextType.SCALAR)`.
+**Fix**: Use `compileNode()` helper with explicit context.
+
+### 2. Missing opcode implementation
+**Symptom**: "Unknown opcode" or silent wrong result.
+**Fix**: Add opcode to `Opcodes.java`, handler to `BytecodeInterpreter.java`, emitter to `BytecodeCompiler.java`, disassembly to `InterpretedCode.java`.
+
+### 3. Closure variable not accessible
+**Symptom**: Variable returns undef inside eval/sub.
+**Pattern**: Variable not registered in symbol table.
+**Fix**: Ensure `detectClosureVariables()` registers captured variables via `addVariableWithIndex()`.
+
+### 4. Double compilation of RHS
+**Symptom**: Side effects happen twice (e.g., `shift` removes two elements).
+**Pattern**: RHS compiled once at top of function, then again in specific handler.
+**Fix**: Remove redundant compilation, use `valueReg` from first compilation.
+
+### 5. Lvalue not preserved
+**Symptom**: Assignment doesn't modify original variable.
+**Pattern**: Expression returns copy instead of lvalue reference.
+**Fix**: Ensure lvalue context is preserved through compilation chain.
+
+### 6. LIST_TO_COUNT destroys value
+**Symptom**: Numeric value instead of expected string/reference.
+**Pattern**: Incorrect scalar context conversion.
+**Fix**: Remove spurious `LIST_TO_COUNT` or use proper scalar coercion.
+
+## Test File Categories
+
+| Directory | Tests | Notes |
+|-----------|-------|-------|
+| `perl5_t/t/op/` | Core operators | bop.t, sprintf.t, etc. |
+| `perl5_t/t/re/` | Regex | pat.t needs special env vars |
+| `perl5_t/t/io/` | I/O operations | filetest.t, etc. |
+| `perl5_t/t/uni/` | Unicode | |
+| `perl5_t/t/mro/` | Method resolution | |
+
+## Commit Message Format
+
+```
+Fix <what> by <how>
+
+<Details of the bug and fix>
+
+Generated with [Devin](https://cli.devin.ai/docs)
+
+Co-Authored-By: Devin <noreply@cognition.ai>
+```
+
+## Quick Reference Commands
+
+```bash
+# Build
+mvn package -q -DskipTests
+
+# Test
+mvn test
+perl dev/tools/perl_test_runner.pl perl5_t/t/op/bop.t
+
+# Debug
+./jperl --disassemble -e 'code'
+./jperl --disassemble --interpreter -e 'code'
+
+# Compare
+diff <(./jperl -e 'code') <(perl -e 'code')
+
+# Bisect
+git log master..HEAD --oneline
+git checkout <sha> && mvn package -q -DskipTests && ./jperl -e 'test'
+```