debug-exiftool

name	debug-exiftool
description	Debug and fix Image::ExifTool test failures in PerlOnJava
argument-hint	[test-name or test-file]
triggers	["user","model"]

⚠️⚠️⚠️ CRITICAL: NEVER USE git stash ⚠️⚠️⚠️

DANGER: Changes are SILENTLY LOST when using git stash/stash pop!

• NEVER use git stash to temporarily revert changes
• INSTEAD: Commit to a WIP branch or use git diff > backup.patch
• This warning exists because completed work was lost during debugging

Debugging Image::ExifTool Tests in PerlOnJava

You are debugging failures in the Image::ExifTool test suite running under PerlOnJava (a Perl-to-JVM compiler/interpreter). Failures typically stem from missing Perl features or subtle behavior differences in PerlOnJava, not bugs in ExifTool itself.

Git Workflow

IMPORTANT: Never push directly to master. Always use feature branches and PRs.

IMPORTANT: Always commit or stash changes BEFORE switching branches. If git stash pop has conflicts, uncommitted changes may be lost.

git checkout -b fix/exiftool-issue-name
# ... make changes ...
git push origin fix/exiftool-issue-name
gh pr create --title "Fix: description" --body "Details"

Project Layout

• PerlOnJava source: src/main/java/org/perlonjava/ (compiler, bytecode interpreter, runtime)
• ExifTool distribution: Image-ExifTool-13.44/ (unmodified upstream)
• ExifTool tests: Image-ExifTool-13.44/t/*.t
• ExifTool test lib: Image-ExifTool-13.44/t/TestLib.pm (exports check, writeCheck, writeInfo, testCompare, binaryCompare, testVerbose, notOK, done)
• ExifTool test data: Image-ExifTool-13.44/t/images/ (reference images)
• ExifTool reference output: Image-ExifTool-13.44/t/<TestName>_N.out (expected tag output per sub-test)
• PerlOnJava unit tests: src/test/resources/unit/*.t (make suite, 154 tests)
• Perl5 core tests: perl5_t/t/ (Perl 5 compatibility suite, run via make test-gradle)
• Fat JAR: target/perlonjava-3.0.0.jar
• Launcher script: ./jperl (resolves JAR path, sets $^X)

Building PerlOnJava

ALWAYS use make commands. NEVER use raw mvn/gradlew commands.

Command	What it does
make	Build + run all unit tests (use before committing)
make dev	Build only, skip tests (for quick iteration during debugging)

make       # Standard build - compiles and runs tests
make dev   # Quick build - compiles only, NO tests

Running ExifTool Tests

Single test

cd Image-ExifTool-13.44
java -jar ../target/perlonjava-3.0.0.jar -Ilib t/Writer.t
# Or using the launcher:
cd Image-ExifTool-13.44
../jperl -Ilib t/Writer.t

Single test with timeout (prevents infinite loops)

cd Image-ExifTool-13.44
timeout 120 java -jar ../target/perlonjava-3.0.0.jar -Ilib t/XMP.t

All ExifTool tests in parallel with summary

cd Image-ExifTool-13.44
mkdir -p /tmp/exiftool_results
for t in t/*.t; do
    name=$(basename "$t" .t)
    ( output=$(timeout 120 java -jar ../target/perlonjava-3.0.0.jar -Ilib "$t" 2>&1)
      ec=$?
      if [ $ec -eq 124 ]; then echo "$name TIMEOUT"
      else
          pass=$(echo "$output" | grep -cE '^ok ')
          fail=$(echo "$output" | grep -cE '^not ok ')
          plan=$(echo "$output" | grep -oE '^1\.\.[0-9]+' | head -1)
          planned=${plan#1..}
          echo "$name pass=$pass fail=$fail planned=${planned:-?} exit=$ec"
      fi
    ) > "/tmp/exiftool_results/$name.txt" &
done
wait
echo "=== RESULTS ==="
cat /tmp/exiftool_results/*.txt | sort
echo "=== TOTALS ==="
cat /tmp/exiftool_results/*.txt | awk '{
    for(i=1;i<=NF;i++) {
        if($i~/^pass=/) p+=substr($i,6)
        if($i~/^fail=/) f+=substr($i,6)
        if($i~/^planned=/) { v=substr($i,9); if(v!="?") pl+=v }
    }
} END { printf "PASS=%d FAIL=%d PLANNED=%d RATE=%d%%\n", p, f, pl, (pl>0?p*100/pl:0) }'

Running Perl5 core tests (e.g. lexsub.t)

cd perl5_t/t
../../jperl op/lexsub.t

Running Perl5 core tests that use subprocess tests

Tests using run_multiple_progs() or fresh_perl_is() spawn jperl as a subprocess. This requires jperl to be in PATH:

# Using the test runner (handles PATH automatically):
perl dev/tools/perl_test_runner.pl perl5_t/t/op/eval.t

# Manual running (must set PATH):
PATH="/Users/fglock/projects/PerlOnJava2:$PATH" cd perl5_t/t && ../../jperl op/eval.t

Comparing with System Perl

When debugging, compare PerlOnJava output with native Perl to isolate the difference:

# Run with system Perl
cd Image-ExifTool-13.44
perl -Ilib t/Writer.t 2>&1 | grep -E '^(not )?ok ' > /tmp/perl_results.txt

# Run with PerlOnJava
java -jar ../target/perlonjava-3.0.0.jar -Ilib t/Writer.t 2>&1 | grep -E '^(not )?ok ' > /tmp/jperl_results.txt

# Diff
diff /tmp/perl_results.txt /tmp/jperl_results.txt

For individual Perl constructs:

# System Perl
perl -e 'my @a = (1,2,3); $_ *= 2 foreach @a; print "@a\n"'

# PerlOnJava
java -jar target/perlonjava-3.0.0.jar -e 'my @a = (1,2,3); $_ *= 2 foreach @a; print "@a\n"'

For comparing .failed output files against .out reference files:

cd Image-ExifTool-13.44
diff t/Writer_11.out t/Writer_11.failed

Environment Variables

Compiler/Interpreter Control

Variable	Effect
JPERL_DISABLE_INTERPRETER_FALLBACK=1	Disable bytecode interpreter fallback for large subs (force JVM compilation only)
JPERL_SHOW_FALLBACK=1	Print a message when a sub falls back to the bytecode interpreter
JPERL_EVAL_NO_INTERPRETER=1	Disable interpreter for eval STRING (force JVM compilation)
JPERL_SPILL_SLOTS=N	Set number of JVM spill slots (default 16)

Debugging/Tracing

Variable	Effect
JPERL_ASM_DEBUG=1	Print JVM bytecode disassembly when ASM frame computation crashes
JPERL_ASM_DEBUG_CLASS=<name>	Filter ASM debug output to a specific generated class name
JPERL_BYTECODE_SIZE_DEBUG=1	Print bytecode size for each generated method
JPERL_EVAL_VERBOSE=1	Verbose error reporting for eval STRING compilation issues
JPERL_EVAL_TRACE=1	Trace eval STRING execution path (compile, interpret, fallback)
JPERL_IO_DEBUG=1	Trace file handle open/dup/write operations
JPERL_STDIO_DEBUG=1	Trace STDOUT/STDERR flush sequencing
JPERL_REQUIRE_DEBUG=1	Trace require/use module loading
JPERL_TRACE_CONTROLFLOW=1	Trace control flow detection (goto, return, last/next/redo safety)
JPERL_DISASSEMBLE=1	Disassemble generated bytecode (also --disassemble CLI flag)

Perl-level

Variable	Effect
JPERL_UNIMPLEMENTED=warn	Downgrade unimplemented regex features from fatal to warning

Usage with jperl launcher

# Pass JVM options via JPERL_OPTS
JPERL_OPTS="-Xmx512m" ./jperl script.pl

# Combine env vars
JPERL_SHOW_FALLBACK=1 JPERL_EVAL_TRACE=1 java -jar target/perlonjava-3.0.0.jar -Ilib t/Writer.t 2>&1

Test File Anatomy

ExifTool .t files follow a common pattern:

BEGIN { $| = 1; print "1..N\n"; require './t/TestLib.pm'; t::TestLib->import(); }
END { print "not ok 1\n" unless $loaded; }
use Image::ExifTool;
$loaded = 1;

# Read test: extract tags and compare against t/<TestName>_N.out
my $exifTool = Image::ExifTool->new;
my $info = $exifTool->ImageInfo('t/images/SomeFile.ext', @tags);
print 'not ' unless check($exifTool, $info, $testname, $testnum);
print "ok $testnum\n";

# Write test: modify tags and verify output
writeInfo($exifTool, 'src.jpg', 'tmp/out.jpg', \@setNewValue_args);

# Binary compare test: verify exact byte-for-byte match
binaryCompare('output.jpg', 't/images/original.jpg');

The check() function compares extracted tags against reference files t/<TestName>_N.out. Failed tests leave t/<TestName>_N.failed files for comparison. The writeInfo() function calls SetNewValue + WriteInfo.

Debugging Workflow

1. Run the failing test and capture full output (stdout + stderr). Look for:

   • not ok N lines (which specific sub-tests fail)
   • Runtime exceptions / stack traces from Java
   • Can't locate ... (missing module)
   • Undefined subroutine / Can't call method errors

2. Identify the failing sub-test number and find it in the .t file. Map it to the ExifTool operation (read vs write, which image format, which tags).

3. Check the .out vs .failed files to understand the difference:

   diff t/Writer_11.out t/Writer_11.failed
   

4. Compare with system Perl to confirm it's a PerlOnJava issue, not a test environment issue.

5. Isolate the Perl construct causing the failure. Write a minimal reproducer:

   java -jar target/perlonjava-3.0.0.jar -e 'print pos("abc" =~ /b/g), "\n"'
   perl -e 'print pos("abc" =~ /b/g), "\n"'
   

6. Trace into PerlOnJava source to find the bug. Use JPERL_SHOW_FALLBACK=1 to check if large subs are hitting the interpreter path.

7. Fix in PerlOnJava, rebuild (make dev), re-run the ExifTool test.

8. Verify no regressions: Run make (154 unit tests) and check perl5_t/t/op/lexsub.t (sensitive to block/sub emission changes).

Interpreter Fallback Architecture

PerlOnJava has two compilation backends:

• JVM backend (default): Compiles Perl AST to JVM bytecode via ASM. Fast, but has a ~64KB method size limit.
• Bytecode interpreter (fallback): When a subroutine is too large for JVM (>N lines, typically ~500), it's compiled to PerlOnJava's own bytecode and interpreted. This includes eval STRING by default.

Key files for the interpreter:

• BytecodeCompiler.java — compiles AST to interpreter bytecode
• BytecodeInterpreter.java — executes interpreter bytecode
• CompileAssignment.java — assignment compilation for interpreter
• Opcodes.java — opcode definitions
• InterpretedCode.java — runtime representation of interpreter-compiled code

Closure variables are the main challenge for the interpreter fallback path. There are two distinct mechanisms:

1. Inner named subs within the large sub: These are compiled by SubroutineParser using the JVM compiler (via compilerSupplier). They get full closure support through RETRIEVE_BEGIN_* opcodes and VariableCollectorVisitor.java.

2. The large sub itself accessing outer-scope my variables: This is handled by detectClosureVariables() in BytecodeCompiler.java. It must:

   • Use getAllVisibleVariables() (TreeMap, sorted by register index) with the exact same filtering as SubroutineParser (skip @_, empty decl, fields, & refs) to ensure the capturedVars ordering matches withCapturedVars().
   • Register captured variables in the compiler's symbol table via addVariableWithIndex() so that ALL variable resolution paths find them — not just visit(IdentifierNode). This is critical because handleHashElementAccess, handleArrayElementAccess, hash slices, array slices, and assignment targets all have their own variable lookup logic that checks the symbol table.
   • Reserve registers (bump nextRegister) so local my declarations don't collide with captured variable registers.
   • Scan AST-referenced non-local variables and add them to capturedVarIndices for register recycling protection (prevents getHighestVariableRegister() from being too low).

The runtime flow for captured variables in the interpreter path:

1. compileToInterpreter() creates BytecodeCompiler, calls compiler.compile(ast, ctx) which runs detectClosureVariables() — this sets up capturedVarIndices (name→register mapping) used during bytecode generation
2. compileToInterpreter() creates placeholder capturedVars (all RuntimeScalar)
3. SubroutineParser.withCapturedVars() replaces the placeholder with actual values from paramList (built from getAllVisibleVariables() with same filtering)
4. At runtime, BytecodeInterpreter.execute() copies capturedVars[i] to registers[3+i] via System.arraycopy
5. The compiled bytecode accesses these registers for captured variable reads/writes

Key invariant: The ordering of variables in detectClosureVariables() MUST match SubroutineParser's paramList ordering, because capturedVars[i] is copied to register 3+i and the bytecode was compiled expecting specific variables at specific registers.

Common Failure Patterns

Infinite loops / TIMEOUT

• Often caused by return inside a block refactored by LargeBlockRefactorer into sub { ... }->(@_). The return exits the anonymous sub instead of the enclosing function.
• Can also be caused by regex catastrophic backtracking.
• Use timeout 120 to prevent hangs; JPERL_SHOW_FALLBACK=1 to see if interpreter fallback is involved.

Missing mandatory EXIF tags on write

• When creating EXIF, mandatory tags (YCbCrPositioning, ExifVersion, ComponentsConfiguration, ColorSpace) should be auto-created by WriteExif.pl using %mandatory hash.
• If these are missing, check that %mandatory is accessible (closure variable issue in interpreter fallback).

Closure variable inaccessibility in interpreter

• File-scope my %hash / my @array not accessible inside large subs compiled by interpreter.
• Symptoms: tags silently missing from output, no error messages. Hash lookups return undef instead of the expected values.
• Root cause pattern: The bytecode compiler has MULTIPLE variable resolution paths (visit(IdentifierNode), handleHashElementAccess, handleArrayElementAccess, hash/array slices, assignment LHS). If captured variables are only in capturedVarIndices but NOT in the compiler's symbol table, most access paths won't find them and fall through to global variable load (which returns an empty hash/array).
• Fix: detectClosureVariables() must call symbolTable.addVariableWithIndex() for each captured variable so all resolution paths find them.
• Debugging: Add System.err.println in BytecodeInterpreter.execute() after the System.arraycopy for capturedVars to verify the correct values are being passed at runtime. Also check the handleHashElementAccess code path to see if it reaches LOAD_GLOBAL_HASH (bad) vs getVariableRegister (good).

XMP lang-alt writing failures

• Non-default language entries (en, de, fr) fail to be created in lang-alt lists.
• Related to WriteXMP.pl path tracking using pos() after m//g regex.

pos() behavior after m//g

• pos() returning wrong value after global regex match can cause index tracking bugs in ExifTool's write logic.

Foreach loop variable aliasing

• Postfix foreach (EXPR foreach @list) must alias $_ to actual array elements for modification.
• Block-form and statement-modifier foreach have different code paths in StatementParser.java vs StatementResolver.java.

Encoding / binary data issues

• ExifTool heavily uses binmode, sysread, syswrite, pack, unpack, Encode::decode/encode.
• BYTE_STRING vs STRING type propagation in concat operations can corrupt binary data.

Read-only variable violations

• Operations that try to modify read-only scalars (e.g., $_ aliased to a constant).

Current Test Status (as of 2026-03-03)

ExifTool Test Results: 590/600 planned (98%)

Test	Pass/Planned	Status
ExifTool.t	35/35	PASS
Writer.t	59/61	2 fail (test 10: Pentax date fmt, test 46: XMP Audio data)
XMP.t	44/54	10 fail
Geotag.t	3/12	9 fail
PDF.t	18/26	8 fail
QuickTime.t	17/22	5 fail
CanonVRD.t	19/24	5 fail
Nikon.t	6/9	3 fail
CanonRaw.t	5/9	3 fail + crash
Pentax.t	1/4	3 fail
Panasonic.t	2/5	3 fail
(72 other tests)	all pass	PASS

Fix Priority (by impact)

P1: Writer.t remaining failures (2 tests: Writer 10, 46)

• Test 10: Pentax MakerNotes date 2008:03:02 becomes 2008:0:0, time 12:01:23 becomes 12:0:0. Binary date decoding issue — likely pack/unpack or BCD decode in Pentax.pm. Also has a float rounding diff (13.2 vs 13.3).
• Test 46: Missing [XMP, XMP-GAudio, Audio] Data - Audio Data: (Binary data 1 bytes) in output. An XMP Audio binary data tag is not being written/preserved.

RESOLVED: Writer.t closure variable fix (previously P1, 15 tests fixed)

The %mandatory and %crossDelete hashes in WriteExif.pl are file-scope my variables accessed inside the large WriteExif sub (compiled by interpreter fallback). Fixed by registering captured variables in the compiler's symbol table via addVariableWithIndex() in detectClosureVariables(). This fixed Writer tests 6,7,11,13,19,25-28,35,38,42,48,53,55.

P2: Geotag date/time computation (9 tests: Geotag 2,4,6-12)

All geotag tests except module loading and 2 others fail. All use Time::Local for date arithmetic and GPS coordinate interpolation. Likely one root cause in date string parsing or timezone offset calculation. Compare Geotag_2.out vs Geotag_2.failed to see if GPS coordinates are wrong or dates are wrong.

P3: XMP lang-alt writing (5 tests: XMP 13,17,26,51,52)

Writing non-default language entries to XMP lang-alt lists fails silently. Only x-default works. The write path in WriteXMP.pl uses pos() after m//g for path tracking. Test with:

perl -e '"a/b/c" =~ m|/|g; print pos(), "\n"'  # should print 2
java -jar target/perlonjava-3.0.0.jar -e '"a/b/c" =~ m|/|g; print pos(), "\n"'

P4: XMP lang-alt Bag index tracking (3 tests: XMP 36,38,50)

Values assigned to wrong bag items; empty strings dropped from lists. Also likely pos() related. Test 36 specifically loses an empty string as first list element.

P5: PDF write/revert cycle (8 tests: PDF 7-12,25,26)

Tests 7-12 are sequential edit/revert operations on a PDF — one failure cascades. Tests 25-26 are AES encryption (require Digest::SHA). Investigate test 7 first as it's the cascade root.

P6: QuickTime write failures (5 tests: QuickTime 11-13,18,20)

HEIC write failures and VideoKeys/AudioKeys extraction. Lower priority — likely format-specific issues.

P7: Other write failures (CanonVRD 5, Nikon 3, Pentax 3, Panasonic 3, etc.)

Various format-specific write issues. Many may share root causes with P1 (mandatory EXIF tags).

Key Source Files Quick Reference

Area	File
Bytecode compiler	backend/bytecode/BytecodeCompiler.java
Bytecode interpreter	backend/bytecode/BytecodeInterpreter.java
Assignment compilation (interp)	backend/bytecode/CompileAssignment.java
Variable collector (closures)	backend/bytecode/VariableCollectorVisitor.java
Opcodes	backend/bytecode/Opcodes.java
Block emission (JVM)	backend/jvm/EmitBlock.java
Subroutine emission (JVM)	backend/jvm/EmitSubroutine.java
Foreach emission (JVM)	backend/jvm/EmitForeach.java
Eval handling (JVM)	backend/jvm/EmitEval.java
Method creator / fallback	backend/jvm/EmitterMethodCreator.java
Large block refactoring	backend/jvm/LargeBlockRefactorer.java
Control flow safety	frontend/analysis/ControlFlowDetectorVisitor.java
Statement parser (block foreach)	frontend/parser/StatementParser.java
Statement resolver (postfix foreach)	frontend/parser/StatementResolver.java
Subroutine parser	frontend/parser/SubroutineParser.java
Runtime scalar	runtime/runtimetypes/RuntimeScalar.java
Runtime array	runtime/runtimetypes/RuntimeArray.java
Runtime hash	runtime/runtimetypes/RuntimeHash.java
Dynamic variables	runtime/runtimetypes/DynamicVariableManager.java
IO operations	runtime/runtimetypes/RuntimeIO.java
IO operator (open/dup)	runtime/operators/IOOperator.java
Control flow (goto/labels)	backend/jvm/EmitControlFlow.java
Dereference / slicing	backend/jvm/Dereference.java
Variable emission (refs)	backend/jvm/EmitVariable.java
String parser (qw, heredoc)	frontend/parser/StringParser.java
String operators	runtime/operators/StringOperators.java
Pack/Unpack	runtime/operators/PackOperator.java
Regex preprocessor	runtime/regex/RegexPreprocessor.java
Regex runtime	runtime/regex/RuntimeRegex.java
Module loading	runtime/operators/ModuleOperators.java

All paths relative to src/main/java/org/perlonjava/.

Lessons Learned (Debugging Pitfalls)

Register recycling inflation

The HEAD code's AST-based detectClosureVariables populated capturedVarIndices with ~321 entries, which inflated getHighestVariableRegister() and prevented aggressive register recycling. A no-op version (removing all capturedVarIndices) dropped Writer.t from 44/61 to 26/61 — not because of closure access, but because register recycling became too aggressive. When modifying detectClosureVariables, always ensure capturedVarIndices has enough entries to keep getHighestVariableRegister() high enough to prevent register corruption.

Multiple variable resolution paths

The bytecode compiler resolves variables in MANY separate code paths:

• visit(IdentifierNode) — checks capturedVarIndices then symbol table
• handleHashElementAccess — checks closure vars, symbol table, then global
• handleArrayElementAccess — same pattern
• handleHashSlice, handleArraySlice, handleHashKeyValueSlice — same
• Assignment targets in CompileAssignment.java — same pattern
• Various places in CompileOperator.java

If a fix only patches ONE of these paths (e.g., capturedVarIndices check in visit(IdentifierNode)), hash/array access will still fall through to globals. The correct fix is to register captured variables in the symbol table so ALL paths find them.

Ordering matters for capturedVars

SubroutineParser builds paramList by iterating getAllVisibleVariables() (TreeMap sorted by register index) with specific filters. detectClosureVariables() must use the exact same iteration order and filters. Any mismatch causes captured variable values to be assigned to wrong registers at runtime.

goto LABEL across JVM scope boundaries

EmitControlFlow.handleGotoLabel() resolves labels at compile time within the current JVM scope. When the target label is outside the current scope (e.g., goto inside a map block to a label outside, or goto inside an eval block), the compile-time lookup fails. The fix is to emit a RuntimeControlFlowList marker with ControlFlowType.GOTO at runtime (the same mechanism used by dynamic goto EXPR), allowing the goto signal to propagate up the call stack. This was a blocker for both op/array.t and op/eval.t.

List slice with range indices

In Dereference.handleArrowArrayDeref(), the check for single-index vs slice path must account for range expressions (.. operator). A range like 0..5 is a single AST node but produces multiple indices. The correct condition is: use single-index path only if there's one element AND it's not a range. Otherwise, use the slice path. The old code had a complex isArrayLiteral check that was too restrictive.

qw() backslash processing

StringParser.parseWordsString() must apply single-quote backslash rules to each word: \\ → \ and \delimiter → delimiter. Without this, backslashes are doubled in the output. The processing uses the closing delimiter from the qw construct.

\(LIST) must flatten arrays before creating refs

\(@array) should create individual scalar refs to each array element (like map { \$_ } @array), not a single ref to the array. EmitVariable needs a flattenElements() method that detects @ sigil nodes in the list and flattens them before creating element references.

Squashing a diverged branch with git diff + git apply

When a feature branch has diverged far from master (thousands of commits in common history), both git rebase and git merge --squash can produce massive conflicts across dozens of files. The clean workaround:

# 1. Generate a patch of ONLY the branch's changes vs master
git diff master..feature-branch > /tmp/branch-diff.patch
# 2. Create a fresh branch from current master
git checkout master && git checkout -b feature-branch-clean
# 3. Apply the patch (no merge history = no conflicts)
git apply /tmp/branch-diff.patch
# 4. Commit as a single squashed commit
git add -A && git commit -m "Squashed: ..."
# 5. Force push to update the PR
git push --force origin feature-branch-clean

This works because git diff master..branch produces the exact file-level delta, bypassing all the intermediate merge history that causes conflicts.

Always commit fixes before rebasing

Uncommitted working tree changes are lost when git rebase --abort is run. If you have a fix in progress (e.g., a BitwiseOperators change), commit it first — even as a WIP commit — before attempting any rebase. The rebase abort restores the branch to its pre-rebase state, which does NOT include uncommitted changes.

getInt() vs (int) getLong() for 32-bit integer wrapping

RuntimeScalar.getInt() clamps DOUBLE values to Integer.MAX_VALUE (e.g., (int) 2147483648.0 == 2147483647). But (int) getLong() wraps correctly via long→int truncation (e.g., (int) 2147483648L == -2147483648). For use integer operations where Config.pm reports ivsize=4, always use (int) getLong() to get proper 32-bit wrapping behavior matching Perl's semantics.

scalar gmtime/localtime ctime(3) format

Perl's scalar gmtime/localtime returns ctime(3) format: "Fri Mar 7 20:13:52 881" — NOT RFC 1123 ("Fri, 7 Mar 0881 20:13:52 GMT"). Use String.format() with explicit field widths, not DateTimeFormatter. Also: wday must use getValue() % 7 (Perl: 0=Sun..6=Sat) not getValue() (Java: 1=Mon..7=Sun). Large years (>9999) must not crash the formatter.

Regression testing: always compare branch vs master

Before declaring a fix complete, run the same test on both master and the branch to distinguish real regressions from pre-existing failures. Use perl5_t/t/ (not perl5/t/) for running Perl5 core tests — the perl5_t copy has test harness files (test.pl, charset_tools.pl) that PerlOnJava can load.

Adding Debug Instrumentation

In ExifTool Perl code (temporary, never commit):

print STDERR "DEBUG: variable=$variable\n";

In PerlOnJava Java code (temporary, never commit):

System.err.println("DEBUG: value=" + value);

To trace which subs hit interpreter fallback:

JPERL_SHOW_FALLBACK=1 java -jar target/perlonjava-3.0.0.jar -Ilib t/Writer.t 2>&1 | grep FALLBACK