PERLHACK(1)

PERLHACK(1) ✨

Perl Programmers Reference Guide — okay bestie, it's time to hack on Perl 💅

NAME 🦋

perlhack — how to hack on Perl, periodt. 💖

DESCRIPTION 🌸

Okay so this whole doc is your guide to how Perl development actually works. It spills the tea on the Perl 5 Porters email list, the Perl repository, the Perl bug tracker, patch guidelines, and some commentary on Perl development philosophy. Basically everything you need to know to slide into Perl core dev. 💅

SUPER QUICK PATCH GUIDE 🔥

If you just wanna submit one tiny patch — like a pod fix, a test for a bug, comment fixes, whatever — it's literally so easy! Here's the whole vibe:

• Check out the source repository 🐙 The perl source lives in a git repository. Clone it like this:

% git clone https://github.com/Perl/perl5.git perl

• Ensure you're following the latest advice 📖 Just in case this guide got updated recently, read the freshest version straight from the perl source:

% perldoc pod/perlhack.pod

• Create a branch for your change 🌿 Make a branch based on blead to commit your change to. Later you'll use it to send your patch to the Perl issue tracker.

% git checkout -b mychange

• Make your change ✨ Hack, hack, hack. But keep in mind: Perl runs on so many different platforms, with different operating systems that have different capabilities, different filesystem organizations, and even different character sets. perlhacktips gives advice on all that.
• Test your change 🧪 Run all the tests with these commands:

% ./Configure -des -Dusedevel
% make test

Keep hacking until the tests pass. No cap.

• Commit your change 💾 Committing saves the change on your local system:

% git commit -a -m 'Commit message goes here'

Make sure the commit message describes your change in a single sentence. For example, "Fixed spelling errors in perlhack.pod".

• Send your change to the Perl issue tracker 📬 Next step: submit your patch to the Perl core ticket system.

Create a GitHub fork of the perl5 repository and add it as a remote, if you haven't already. The GitHub documentation at https://help.github.com/en/articles/working-with-forks explains how.

% git remote add fork git@github.com:MyUser/perl5.git

For more info, see "Connecting to GitHub with SSH" at https://docs.github.com/en/free-pro-team@latest/github/authenticating-to-github/connecting-to-github-with-ssh.

If you'd rather use an HTTPS URL for your git push, see "Cloning with HTTPS URLs" at https://docs.github.com/en/free-pro-team@latest/github/using-git/which-remote-url-should-i-use#cloning-with-https-urls.

% git remote add fork https://github.com/MyUser/perl5.git

Then push your new branch to your fork.

% git push -u fork mychange

Finally, create a Pull Request on GitHub from your branch to blead. The GitHub documentation at https://help.github.com/en/articles/creating-a-pull-request-from-a-fork walks you through it.

• Thank you 🫶 The porters appreciate the time you spent helping make Perl better. Thank you!
• Acknowledgement 👑 All contributors are credited (by name and email address) in the AUTHORS file, which is part of the perl distribution, as well as the Git commit history.

If you don’t want to be included in the AUTHORS file, just let us know. Otherwise we'll take your submission of a patch as permission to credit you in the AUTHORS file.

• Next time 🔁 Next time you wanna make a patch, start from the latest perl in a pristine state. Check you don't have any local changes or added files in your perl check-out that you wanna keep, then run these commands:

% git checkout blead
% git pull
% git reset --hard origin/blead
% git clean -dxf

BUG REPORTING 🐛

If you wanna report a bug in Perl, or browse existing Perl bugs and patches, hit up the GitHub issue tracker at https://github.com/perl/perl5/issues.

Please check the archive of the perl5-porters list (see below) and/or the bug tracking system before submitting a bug report. Often you'll find the bug has already been reported. No need to duplicate, bestie.

You can log in to the bug tracking system and comment on existing bug reports. If you have extra info about an existing bug, please add it. It helps the porters fix the bug. 💖

PERL 5 PORTERS 👑

The perl5-porters (p5p) mailing list is where the Perl standard distribution is maintained and developed. The people who maintain Perl are also called the "Perl 5 Porters", "p5p", or just the "porters".

A searchable archive of the list is at https://markmail.org/search/?q=perl5-porters. There's also an archive at https://archive.develooper.com/perl5-porters@perl.org/.

perl-changes mailing list 📬

The perl5-changes mailing list gets a copy of each patch submitted to the maintenance and development branches of the perl repository. See https://lists.perl.org/list/perl5-changes.html for subscription and archive info.

#p5p on IRC 💬

Many porters are also active on the irc://irc.perl.org/#p5p channel. Feel free to join and ask questions about hacking on the Perl core. We don't bite. 🦋

GETTING THE PERL SOURCE 🌈

All of Perl's source code is kept centrally in a Git repository at github.com. The repository contains many Perl revisions from Perl 1 onwards and all the revisions from Perforce, the previous version control system.

For way more detail on using git with the Perl repository, please see perlgit.

Read access via Git 🐙

You'll need a copy of Git on your computer. You can fetch a copy of the repository using the git protocol:

% git clone git@github.com:Perl/perl5.git perl

This clones the repository and makes a local copy in the perl directory.

If you can't use the git protocol because of firewall reasons, you can also clone via http:

% git clone https://github.com/Perl/perl5.git perl

Read access via the web 🌐

You may access the repository over the web. This lets you browse the tree, see recent commits, subscribe to repository notifications, search for particular commits, and more. You may access it at https://github.com/Perl/perl5.

Write access via git ✍️

If you have a commit bit, please see perlgit for more details on using git.

PATCHING PERL 🛠️

If you're planning to do more extensive work than a single small fix, we encourage you to read the documentation below. It'll help you focus your work and make your patches easier to incorporate into the Perl source.

Submitting patches 📥

If you have a small patch to submit, please submit it via the GitHub Pull Request workflow. You may also send patches to the p5p list.

Patches are reviewed and discussed on GitHub or the p5p list. Simple, uncontroversial patches will usually be applied without any discussion. When the patch is applied, the ticket will be updated and you'll receive email.

In other cases, the patch will need more work or discussion. You're encouraged to participate in the discussion and advocate for your patch. Sometimes your patch may get lost in the shuffle. It's appropriate to send a reminder email to p5p if no action has been taken in a month. Please remember the Perl 5 developers are all volunteers, and be polite. 💅

Changes are always applied directly to the main development branch, called "blead". Some patches may be backported to a maintenance branch. If you think your patch is appropriate for the maintenance branch (see "MAINTENANCE BRANCHES" in perlpolicy), please explain why when you submit it.

Getting your patch accepted ✅

If you're submitting a code patch, there are several things you can do to help the Perl 5 Porters accept your patch.

Patch style 🎨

Using the GitHub Pull Request workflow, your patch will automatically be available in a suitable format. If you wish to submit a patch to the p5p list for review, make sure to create it appropriately.

If you used git to check out the Perl source, then using git format-patch will produce a patch in a style suitable for Perl. The format-patch command produces one patch file for each commit you made. If you prefer to send a single patch for all commits, you can use git diff.

% git checkout blead
% git pull
% git diff blead my-branch-name

This produces a patch based on the difference between blead and your current branch. It's important to make sure blead is up to date before producing the diff — that's why we call git pull first.

We strongly recommend using git if possible. It'll make your life easier, and ours as well.

However, if you're not using git, you can still produce a suitable patch. You'll need a pristine copy of the Perl source to diff against. The porters prefer unified diffs. Using GNU diff, you can produce a diff like this:

% diff -Npurd perl.pristine perl.mine

Make sure you make realclean in your copy of Perl to remove any build artifacts, or you may get a confusing result.

Commit message 💌

As you craft each patch you intend to submit to the Perl core, it's important to write a good commit message. This is especially important if your submission will consist of a series of commits.

The first line of the commit message should be a short description without a period. It should be no longer than the subject line of an email — 50 characters is a good rule of thumb.

A lot of Git tools (Gitweb, GitHub, git log --pretty=oneline, ...) will only display the first line (cut off at 50 characters) when presenting commit summaries.

The commit message should include a description of the problem that the patch corrects or new functionality that the patch adds.

As a general rule of thumb, your commit message should help a programmer who knows the Perl core quickly understand what you were trying to do, how you were trying to do it, and why the change matters to Perl.

• Why 🤔 Your commit message should describe why the change you're making is important. When someone looks at your change in six months or six years, your intent should be clear.

If you're deprecating a feature with the intent of later simplifying another bit of code, say so. If you're fixing a performance problem or adding a new feature to support some other bit of the core, mention that.

• What 📝 Your commit message should describe what part of the Perl core you're changing and what you expect your patch to do.
• How 🔧

okay so it's not strictly required for doc changes, new tests, or little baby trivial patches — but bestie, it is so worth explaining how your change actually works. Even if it's crystal clear to you right this second, it might be giving absolutely nothing to a porter next month, or next year. 🫶

And no cap: a commit message is not a stand-in for comments in your code. They do different jobs, fr fr. Commit messages describe the change you made. Code comments describe the current state of the code. Two different vibes, two different jobs. 💅

If you just shipped a whole new feature — complete with docs, tests and well-commented code — a brief commit message is lowkey plenty. But if you changed, like, one single character deep down in the parser or the lexer? Yeah you might need to write a small novel so future readers get what you did and why you did it. 📖

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✨ Comments, Comments, Comments ✨

Please comment your code adequately. Commenting every single line is unnecessary, obviously, but anything that:

• takes advantage of side effects of operators,
• creates changes that will be felt outside of the function being patched,
• or that others might find confusing,

…should be documented. And if you're gonna err? Err on the side of too many comments rather than too few. 🌸

The best comments explain why the code does what it does — not what it does. iykyk. 💖

✨ Style ✨

Generally: follow the particular style of the code you're patching. Match the energy of the file you're in, bestie. 💫

In particular, follow these general guidelines for patching Perl sources:

• 4-wide indents for code, 2-wide indents for nested CPP #defines, with 8-wide tabstops.
• Use spaces for indentation, not tab characters. The codebase is a mixture of tabs and spaces for indentation, and we are moving to spaces only. Converting lines you're patching from 8-wide tabs to spaces will help this migration.
• Try not to exceed 79 columns. In general, we target 80 column lines. When sticking to 80 columns would lead to torturous code or rework, it's fine to go longer. Try to keep your excess past 80 to a minimum.
• ANSI C prototypes.
• Uncuddled elses and "K&R" style for indenting control constructs.
• No C++ style (//) comments.
• Mark places that need to be revisited with XXX (and revisit often!).
• Opening brace lines up with if when conditional spans multiple lines; should be at end-of-line otherwise.
• In function definitions, name starts in column 0 (return value-type is on previous line).
• Single space after keywords that are followed by parens, no space between function name and following paren.
• Avoid assignments in conditionals, but if they're unavoidable, use extra paren, e.g. "if (a && (b = c)) ...".
• "return foo;" rather than "return(foo);".
• "if (!foo) ..." rather than "if (foo == FALSE) ..." etc.
• Do not declare variables using "register". It may be counterproductive with modern compilers, and is deprecated in C++, under which the Perl source is regularly compiled.
• In-line functions that are in headers that are accessible to XS code need to be able to compile without warnings with commonly used extra compilation flags, such as gcc's "-Wswitch-default" which warns whenever a switch statement does not have a "default" case. The use of these extra flags is to catch potential problems in legal C code, and is often used by Perl aggregators, such as Linux distributors.

✨ Test suite ✨

If your patch changes code (rather than just changing documentation), you should also include one or more test cases which illustrate the bug you're fixing or validate the new functionality you're adding. In general, you should update an existing test file rather than create a new one. 🧚

Your test suite additions should generally follow these guidelines (courtesy of Gurusamy Sarathy <gsar@activestate.com>):

• Know what you're testing. Read the docs, and the source.
• Tend to fail, not succeed.
• Interpret results strictly.
• Use unrelated features (this will flush out bizarre interactions).
• Use non-standard idioms (otherwise you are not testing TIMTOWTDI).
• Avoid using hardcoded test numbers whenever possible (the EXPECTED/GOT found in t/op/tie.t is much more maintainable, and gives better failure reports).
• Give meaningful error messages when a test fails.
• Avoid using qx// and system() unless you are testing for them. If you do use them, make sure that you cover _all_ perl platforms.
• Unlink any temporary files you create.
• Promote unforeseen warnings to errors with $SIG{__WARN__}.
• Be sure to use the libraries and modules shipped with the version being tested, not those that were already installed.
• Add comments to the code explaining what you are testing for.
• Make updating the '1..42' string unnecessary. Or make sure that you update it.
• Test _all_ behaviors of a given operator, library, or function.
  • Test all optional arguments.
  • Test return values in various contexts (boolean, scalar, list, lvalue).
  • Use both global and lexical variables.
  • Don't forget the exceptional, pathological cases.

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✨ Patching a core module ✨

This works just like patching anything else, with one extra consideration. 👑

Modules in the cpan/ directory of the source tree are maintained outside of the Perl core. When the author updates the module, the updates are simply copied into the core. See that module's documentation or its listing on <https://metacpan.org/> for more information on reporting bugs and submitting patches.

In most cases, patches to modules in cpan/ should be sent upstream and should not be applied to the Perl core individually. If a patch to a file in cpan/ absolutely cannot wait for the fix to be made upstream, released to CPAN and copied to blead, you must add (or update) a "CUSTOMIZED" entry in the Porting/Maintainers.pl file to flag that a local modification has been made. See Porting/Maintainers.pl for more details.

In contrast, modules in the dist/ directory are maintained in the core. 🎀

✨ Updating perldelta ✨

For changes significant enough to warrant a pod/perldelta.pod entry, the porters will greatly appreciate it if you submit a delta entry along with your actual change. Significant changes include, but are not limited to:

• Adding, deprecating, or removing core features
• Adding, deprecating, removing, or upgrading core or dual-life modules
• Adding new core tests
• Fixing security issues and user-visible bugs in the core
• Changes that might break existing code, either on the perl or C level
• Significant performance improvements
• Adding, removing, or significantly changing documentation in the pod/ directory
• Important platform-specific changes

Please make sure you add the perldelta entry to the right section within pod/perldelta.pod. More information on how to write good perldelta entries is available in the "Style" section of Porting/how_to_write_a_perldelta.pod. ✨

✨ What makes for a good patch? ✨

New features and extensions to the language can be contentious. There is no specific set of criteria which determine what features get added, but here are some questions to consider when developing a patch: 💭

Does the concept match the general goals of Perl?

Our goals include, but are not limited to:

1. Keep it fast, simple, and useful.
2. Keep features/concepts as orthogonal as possible.
3. No arbitrary limits (platforms, data sizes, cultures).
4. Keep it open and exciting to use/patch/advocate Perl everywhere.
5. Either assimilate new technologies, or build bridges to them.

Where is the implementation?

All the talk in the world is useless without an implementation. In almost every case, the person or people who argue for a new feature will be expected to be the ones who implement it. Porters capable of coding new features have their own agendas, and are not available to implement your (possibly good) idea. 😭

Backwards compatibility

It's a cardinal sin to break existing Perl programs. New warnings can be contentious — some say that a program that emits warnings is not broken, while others say it is. Adding keywords has the potential to break programs, changing the meaning of existing token sequences or functions might break programs.

The Perl 5 core includes mechanisms to help porters make backwards incompatible changes more compatible such as the feature and deprecate modules. Please use them when appropriate. 🫶

Could it be a module instead?

Perl 5 has extension mechanisms, modules and XS, specifically to avoid the need to keep changing the Perl interpreter. You can write modules that export functions, you can give those functions prototypes so they can be called like built-in functions, you can even write XS code to mess with the runtime data structures of the Perl interpreter if you want to implement really complicated things.

Whenever possible, new features should be prototyped in a CPAN module before they will be considered for the core. 🌈

Is the feature generic enough?

Is this something that only the submitter wants added to the language, or is it broadly useful? Sometimes, instead of adding a feature with a tight focus, the porters might decide to wait until someone implements the more generalized feature.

Does it potentially introduce new bugs?

Radical rewrites of large chunks of the Perl interpreter have the potential to introduce new bugs. 💀

How big is it?

The smaller and more localized the change, the better. Similarly, a series of small patches is greatly preferred over a single large patch.

Does it preclude other desirable features?

A patch is likely to be rejected if it closes off future avenues of development. For instance, a patch that placed a true and final interpretation on prototypes is likely to be rejected because there are still options for the future of prototypes that haven't been addressed.

Is the implementation robust?

Good patches (tight code, complete, correct) stand more chance of going in. Sloppy or incorrect patches might be placed on the back burner until fixes can be made, or they might be discarded altogether without further notice. 🙈

Is the implementation generic enough to be portable?

The worst patches make use of system-specific features. It's highly unlikely that non-portable additions to the Perl language will be accepted.

Is the implementation tested?

Patches which change behaviour (fixing bugs or introducing new features) must include regression tests to verify that everything works as expected. 🎀

Without tests provided by the original author, how can anyone else changing perl in the future be sure that they haven't unwittingly broken the behaviour the patch implements? And without tests, how can the patch's author be confident that his/her hard work put into the patch won't be accidentally thrown away by someone in the future? Like, be so for real rn. 😭

Is there enough documentation?

Patches without documentation are probably ill-thought out or incomplete. No features can be added or changed without documentation, so submitting a patch for the appropriate pod docs as well as the source code is important. Yes, the docs too, bestie, don't ghost them. 💖

Is there another way to do it?

Larry said "Although the Perl Slogan is There's More Than One Way to Do It, I hesitate to make 10 ways to do something". This is a tricky heuristic to navigate, though--one man's essential addition is another man's pointless cruft. iykyk 💅

Does it create too much work?

Work for the committers, work for Perl programmers, work for module authors, ... Perl is supposed to be easy. periodt. ✨

Patches speak louder than words

Working code is always preferred to pie-in-the-sky ideas. A patch to add a feature stands a much higher chance of making it to the language than does a random feature request, no matter how fervently argued the request might be. This ties into "Will it be useful?", as the fact that someone took the time to make the patch demonstrates a strong desire for the feature. show, don't tell 🫶

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🧪 TESTING 🧪

The core uses the same testing style as the rest of Perl, a simple ok/not ok run through Test::Harness, but there are a few special considerations. Not me warning you about the ✨vibes✨ in advance. 💖

There are three ways to write a test in the core: Test::More, t/test.pl and ad hoc print $test ? "ok 42\n" : "not ok 42\n". The decision of which to use depends on what part of the test suite you're working on. This is a measure to prevent a high-level failure (such as Config.pm breaking) from causing basic functionality tests to fail. lowkey genius tbh 🧠

The t/test.pl library provides some of the features of Test::More, but avoids loading most modules and uses as few core features as possible. it's giving minimal, it's giving unbothered. 🦋

If you write your own test, use the Test Anything Protocol (https://testanything.org).

• t/base, t/comp and t/opbasic

Since we don't know if require works, or even subroutines, use ad hoc tests for these three. Step carefully to avoid using the feature being tested. Tests in t/opbasic, for instance, have been placed there rather than in t/op because they test functionality which t/test.pl presumes has already been demonstrated to work. 🩰

• All other subdirectories of t/

Now that basic require() and subroutines are tested, you can use the t/test.pl library.

You can also use certain libraries like Config conditionally, but be sure to skip the test gracefully if it's not there. no crashing out pls 🙈

• Test files not found under t/

This category includes .t files underneath directories such as dist, ext and lib. Since the core of Perl has now been tested, Test::More can and now should be used. You can also use the full suite of core modules in the tests. (As noted in "Patching a core module" above, changes to .t files found under cpan/ should be submitted to the upstream maintainers of those modules.) 👑

When you say make test, Perl uses the t/TEST program to run the test suite (except under Win32 where it uses t/harness instead). All tests are run from the t/ directory, not the directory which contains the test. This causes some problems with the tests in lib/, so here's some opportunity for some patching. 💅

You must be triply conscious of cross-platform concerns. This usually boils down to using File::Spec, avoiding things like fork() and system() unless absolutely necessary, and not assuming that a given character has a particular ordinal value (code point) or that its UTF-8 representation is composed of particular bytes.

There are several functions available to specify characters and code points portably in tests. The always-preloaded functions utf8::unicode_to_native() and its inverse utf8::native_to_unicode() take code points and translate appropriately. The file t/charset_tools.pl has several functions that can be useful. It has versions of the previous two functions that take strings as inputs -- not single numeric code points: uni_to_native() and native_to_uni(). If you must look at the individual bytes comprising a UTF-8 encoded string, byte_utf8a_to_utf8n() takes as input a string of those bytes encoded for an ASCII platform, and returns the equivalent string in the native platform. For example, byte_utf8a_to_utf8n("\xC2\xA0") returns the byte sequence on the current platform that form the UTF-8 for "U+00A0", since "\xC2\xA0" are the UTF-8 bytes on an ASCII platform for that code point. This function returns "\xC2\xA0" on an ASCII platform, and "\x80\x41" on an EBCDIC 1047 one. slay for portability 💜

But easiest is, if the character is specifiable as a literal, like "A" or "%", to use that; if not so specificable, you can use "\N{}" , if the side effects aren't troublesome. Simply specify all your characters in hex, using "\N{U+ZZ}" instead of "\xZZ". "\N{}" is the Unicode name, and so it always gives you the Unicode character. "\N{U+41}" is the character whose Unicode code point is 0x41, hence is 'A' on all platforms. The side effects are:

• These select Unicode rules. That means that in double-quotish strings, the string is always converted to UTF-8 to force a Unicode interpretation (you can utf8::downgrade() afterwards to convert back to non-UTF8, if possible). In regular expression patterns, the conversion isn't done, but if the character set modifier would otherwise be "/d", it is changed to "/u".
• If you use the form "\N{character name}", the charnames module gets automatically loaded. This may not be suitable for the test level you are doing. ⚠️

If you are testing locales (see perllocale), there are helper functions in t/loc_tools.pl to enable you to see what locales there are on the current platform. 🌍

🎀 Special make test targets 🎀

There are various special make targets that can be used to test Perl slightly differently than the standard test target. Not all them are expected to give a 100% success rate. Many of them have several aliases, and many of them are not available on certain operating systems. set your expectations accordingly, bestie 🌸

• test_porting

This runs some basic sanity tests on the source tree and helps catch basic errors before you submit a patch. your pre-submit glow-up check ✨

• minitest

Run miniperl on t/base, t/comp, t/cmd, t/run, t/io, t/op, t/uni and t/mro tests.

miniperl is a minimalistic perl built to bootstrap building extensions, utilities, documentation etc. It doesn't support dynamic loading and depending on the point in the build process will only have access to a limited set of core modules. miniperl is not intended for day to day use. it's giving "bootstrap era" 💀

• test.valgrind check.valgrind

(Only in Linux) Run all the tests using the memory leak + naughty memory access tool "valgrind". The log files will be named testname.valgrind. naughty memory access?? jail. 🚨

• test_harness

Run the test suite with the t/harness controlling program, instead of t/TEST. t/harness is more sophisticated, and uses the Test::Harness module, thus using this test target supposes that perl mostly works. The main advantage for our purposes is that it prints a detailed summary of failed tests at the end. Also, unlike t/TEST, it doesn't redirect stderr to stdout.

Note that under Win32 t/harness is always used instead of t/TEST, so there is no special "test_harness" target.

Under the Unix build process you may use the TEST_ARGS and TEST_FILES parameters to pass arguments through to the underlying harness call. This means that for instance you could do

make test_harness TEST_ARGS="-v -re pat"

which would make, and then run the test harness in verbose mode over files which contain "pat". Or you could do

make test_harness TEST_ARGS="-torture" TEST_FILES="op/*.t"

and run torture tests on files matching the glob "op/*.t".

Under Win32's "test" target you may use the TEST_SWITCHES and TEST_FILES environment variables to control the behaviour of t/harness. This means you can say

nmake test TEST_FILES="op/*.t"
nmake test TEST_SWITCHES="-torture" TEST_FILES="op/*.t"

Note that for compatibility with the unix build process TEST_ARGS may also be used instead of the traditional TEST_SWITCHES argument. so many ways to say the same thing, iconic 💅

• test-notty test_notty

Sets PERL_SKIP_TTY_TEST to true before running normal test.

🌈 Parallel tests 🌈

The core distribution can now run its regression tests in parallel on Unix-like and Windows platforms. On Unix, instead of running make test, set "TEST_JOBS" in your environment to the number of tests to run in parallel, and run make test_harness. On a Bourne-like shell, this can be done as

TEST_JOBS=3 make test_harness  # Run 3 tests in parallel

An environment variable is used, rather than parallel make itself, because TAP::Harness needs to be able to schedule individual non-conflicting test scripts itself, and there is no standard interface to "make" utilities to interact with their job schedulers.

Tests are normally run in a logical order, with the sanity tests first, then the main tests of the Perl core functionality, then the tests for the non-core modules. On many-core systems, this may not use the hardware as effectively as possible. By also specifying

TEST_JOBS=19 PERL_TEST_HARNESS_ASAP=1 make -j19 test_harness

you signal that you want the tests to finish in wall-clock time as short as possible. After the sanity tests are completed, this causes the remaining ones to be packed into the available cores as tightly as we know how. This has its greatest effect on slower, many-core systems. Throughput was sped up by 20% on an outmoded 24-core system; less on more recent faster ones with fewer cores. numbers!! 🔥

Note that the command line above added a "-j" parameter to make, so as to cause parallel compilation. This may or may not work on your platform. 🙈

Normally data on how long tests take is stored in t/test_state, however you can change this to use a different filename by setting the "PERL_TEST_STATE_FILE" environment variable to something different, or to a false value (0 or the empty string) to disable use of the state mechanism entirely. There are no protections against the format of the state file changing over time, so if you have any issues related to this file it is up to you to delete the file manually and then let the harness recreate it, although the file format does not change frequently so this should not be necessary very often. 🫶

Running tests by hand 💅

ok so you can absolutely just run part of the test suite by hand, bestie. from the t/ directory, do one of these:

./perl -I../lib TEST list-of-.t-files

or

./perl -I../lib harness list-of-.t-files

and if you don't pass any test scripts at all? the whole test suite runs. yeah. all of it. hydrate first. 🫶

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Using t/harness for testing ✨

if you go the harness route you get a lil menu of command line options, and they're listed here in the order they have to appear if you're using them together. important. the order matters.

harness -v -torture -re=pattern LIST OF FILES TO TEST
harness -v -torture -re LIST OF PATTERNS TO MATCH

if LIST OF FILES TO TEST is left out, the file list comes from the manifest instead. and yes, the file list can include shell wildcards — they get expanded out. 🌸

• -v — runs the tests in verbose mode so you can actually see which tests ran, plus any debug output. it's giving transparency.
• -torture — runs the torture tests on top of the normal set. yes queen, suffer a little.
• -re=PATTERN — filters the file list so every test file that runs matches PATTERN. heads up: this form is different from the -re LIST OF PATTERNS form below, because this one lets you supply the file list too.
• -re LIST OF PATTERNS — filters the file list so every test file that runs matches /(LIST|OF|PATTERNS)/. with this form the patterns get joined by |, and you cannot supply a list of files — the test files come from the MANIFEST instead.

you can also run a single test with something like

./perl -I../lib path/to/foo.t

except the harnesses set up some environment variables that might affect how the test executes. so like, not exactly the same vibe:

• PERL_CORE=1 — says "hey, we're running this test as part of the perl core test suite." super useful for modules that have a dual life on CPAN. 💖
• PERL_DESTRUCT_LEVEL=2 — gets set to 2 if it isn't already set (see "PERL_DESTRUCT_LEVEL" in perlhacktips).
• PERL — (only used by t/TEST) if set, it overrides the path to the perl executable used to run the tests. the default is ./perl.
• PERL_SKIP_TTY_TEST — if set, it tells things to skip the tests that need a terminal. it's actually set automatically by the Makefile, but you can force it artificially by running make test_notty.

Other environment variables that may influence tests 🦋

• PERL_TEST_Net_Ping — setting this runs all the Net::Ping module tests; otherwise some tests that interact with the outside world get skipped. see perl58delta.
• PERL_TEST_NOVREXX — setting this skips the vrexx.t tests for OS2::REXX.
• PERL_TEST_NUMCONVERTS — this sets a variable in op/numconvert.t.
• PERL_TEST_MEMORY — setting this includes the tests in t/bigmem/. set it to the number of gigabytes of memory available for testing — like PERL_TEST_MEMORY=4 means tests requiring 4GiB of available memory can be run safely. 🔥

also go read the docs for the Test and Test::Harness modules, they've got more environment variables that affect testing. iykyk.

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Performance testing 🏃♀️

the file t/perf/benchmarks holds snippets of perl code that are meant to be benchmarked across a range of perls by the Porting/bench.pl tool. so if you fix or level-up a performance issue, you might wanna drop a representative code sample into that file, then run bench.pl against the previous and current perls to see what difference it made — and whether anything else slowed down as a consequence. 🫣

the file t/perf/opcount.t exists to test whether a particular code snippet has been compiled into an optree containing specified numbers of particular op types. this is iconic for testing whether optimisations that alter ops — like converting an aelem op into an aelemfast op — are actually doing that.

the files t/perf/speed.t and t/re/speed.t are built to test things that run thousands of times slower if a particular optimisation is broken (for example, the utf8 length cache on long utf8 strings). the move: add a test that takes a fraction of a second normally, and minutes otherwise — which makes the test file time out on failure. 💀 sneaky but effective.

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Building perl at older commits 🧚

while hacking on the Perl core distribution, you may have occasion to configure, build and test perl at an old commit. and sometimes make will just... fail during that process. 🙈 if that happens, you may be able to salvage the situation by using the Devel::PatchPerl library from CPAN (not included in the core) to bring the source code at that commit to a buildable state.

here's a real world example, taken from work done to resolve [perl #10118](https://github.com/Perl/perl5/issues/10118). using Porting/bisect.pl had identified commit ba77e4cc9d1ceebf472c9c5c18b2377ee47062e6 as the commit where a bug got corrected. to confirm, a P5P developer wanted to configure and build perl at commit ba77e4c^ (presumably "bad") and then at ba77e4c (presumably "good"). normal configuration and build was attempted:

$ sh ./Configure -des -Dusedevel
$ make test_prep

make, however, failed with output (excerpted) like this:

cc -fstack-protector -L/usr/local/lib -o miniperl \
  gv.o toke.o perly.o pad.o regcomp.o dump.o util.o \
  mg.o reentr.o mro.o hv.o av.o run.o pp_hot.o sv.o \
  pp.o scope.o pp_ctl.o pp_sys.o doop.o doio.o regexec.o \
  utf8.o taint.o deb.o universal.o globals.o perlio.o \
  numeric.o mathoms.o locale.o pp_pack.o pp_sort.o  \
  miniperlmain.o opmini.o perlmini.o
pp.o: In function `Perl_pp_pow':
pp.c:(.text+0x2db9): undefined reference to `pow'
...
collect2: error: ld returned 1 exit status
makefile:348: recipe for target 'miniperl' failed
make: *** [miniperl] Error 1

another P5P contributor recommended installing and using Devel::PatchPerl for this situation — first to figure out the version of perl at the commit in question, then to patch the source code at that point so a build can happen.

$ perl -MDevel::PatchPerl -e \
    'print Devel::PatchPerl->determine_version("/path/to/sourcecode"),
           "\n";'
5.11.1
$ perl -MDevel::PatchPerl -e \
    'Devel::PatchPerl->patch_source("5.11.1", "/path/to/sourcecode");'

once the source was patched, ./Configure and make test_prep were called and completed successfully, enabling confirmation of the findings in RT #72414. 🎀 slay.

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✨ MORE READING FOR GUTS HACKERS ✨

ok so to hack on the Perl guts you're gonna need to read the following things. no skipping, bestie, this is the syllabus:

• perlsource — an overview of the Perl source tree. this helps you find the files you're looking for. 🔍
• perlinterp — an overview of the Perl interpreter source code, plus some details on how Perl does what it does.
• perlhacktut — this one walks through creating a small patch to Perl's C code. if you're just getting started with Perl core hacking, this will help you understand how it works.
• perlhacktips — more details on hacking the Perl core. this doc focuses on lower level stuff like how to write tests, compilation issues, portability, debugging, etc.
  • if you plan on doing serious C hacking, make sure to read this. 👑
• perlguts — this is of paramount importance, because it's the documentation of what goes where in the Perl source. read it over a couple of times and it might start to make sense — don't worry if it doesn't yet, because the best way to study it is to read it alongside poking at Perl source, and we'll do that later on.
  • Gisle Aas's "illustrated perlguts", also known as illguts, has very helpful pictures:
  • <https://metacpan.org/release/RURBAN/illguts-0.49>
• perlxstut and perlxs — a working knowledge of XSUB programming is incredibly useful for core hacking; XSUBs use techniques drawn from the PP code, which is the portion of the guts that actually executes a Perl program. it's a lot gentler to learn those techniques from simple examples and explanation than from the core itself.
• perlapi — the documentation for the Perl API explains what some of the internal functions do, as well as the many macros used in the source.
• Porting/pumpkin.pod — a collection of words of wisdom for a Perl porter; some of it is only useful to the pumpkin holders, but most of it applies to anyone wanting to go about Perl development. 🎃

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🧪 CPAN TESTERS AND PERL SMOKERS 🌈

the CPAN testers ( <https://cpantesters.org/> ) are a group of volunteers who test CPAN modules on a variety of platforms.

Perl Smokers ( <https://www.nntp.perl.org/group/perl.daily-build/> and <https://www.nntp.perl.org/group/perl.daily-build.reports/> ) automatically test Perl source releases on platforms with various configurations.

both efforts welcome volunteers. 🫶 to get involved in smoke testing of the perl itself, visit <https://metacpan.org/release/Test-Smoke>. and to start smoke testing CPAN modules, visit <https://metacpan.org/release/CPANPLUS-YACSmoke> or <https://metacpan.org/release/minismokebox> or <https://metacpan.org/release/CPAN-Reporter>.

✨ WHAT NEXT? ✨

Okay so you read literally everything in this doc AND all the other docs listed up there?? bestie you're not just ready to hack on Perl, you're ready ready 💅

Here's some more recs to send you off:

• Subscribe to perl5-porters, follow the patches, and try and understand them. And do not be afraid to ask if there's a portion you're not clear on — who knows, you may unearth a bug in the patch... 👀 (yes, the patch. iconic behavior honestly.)
• Do read the README associated with your operating system, e.g. README.aix on the IBM AIX OS. And don't hesitate to supply patches to that README if you find anything missing or changed over a new OS release — that README is not gonna update itself, bestie 🫶
• Find an area of Perl that seems interesting to you, and see if you can work out how it works. Scan through the source, and step over it in the debugger. Play, poke, investigate, fiddle! You'll probably get to understand not just your chosen area but a much wider range of perl's activity as well — and probably sooner than you'd think. no cap. 🦋

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🛣️ "The Road goes ever on and on, down from the door where it began." 🛣️

If you can do these things, you've started on the long road to Perl porting. Thanks for wanting to help make Perl better — and happy hacking! 🌸

📖 Metaphoric Quotations 📖

If you recognized the quote about the Road above, you're in luck. 🍀

Most software projects begin each file with a literal description of each file's purpose. Perl instead begins each with a literary allusion to that file's purpose. That's just the vibe here.

Like chapters in many books, all top-level Perl source files (along with a few others here and there) begin with an epigrammatic inscription that alludes, indirectly and metaphorically, to the material you're about to read. so it's giving... literature major energy, but make it C.

Quotations are taken from writings of J.R.R. Tolkien pertaining to his Legendarium, almost always from The Lord of the Rings. Chapters and page numbers are given using the following editions:

• The Hobbit, by J.R.R. Tolkien. The hardcover, 70th-anniversary edition of 2007 was used, published in the UK by Harper Collins Publishers and in the US by the Houghton Mifflin Company.
• The Lord of the Rings, by J.R.R. Tolkien. The hardcover, 50th-anniversary edition of 2004 was used, published in the UK by Harper Collins Publishers and in the US by the Houghton Mifflin Company.
• The Lays of Beleriand, by J.R.R. Tolkien and published posthumously by his son and literary executor, C.J.R. Tolkien, being the 3rd of the 12 volumes in Christopher's mammoth History of Middle Earth. Page numbers derive from the hardcover edition, first published in 1983 by George Allen & Unwin; no page numbers changed for the special 3-volume omnibus edition of 2002 or the various trade-paper editions, all again now by Harper Collins or Houghton Mifflin.

Other JRRT books fair game for quotes would thus include The Adventures of Tom Bombadil, The Silmarillion, Unfinished Tales, and The Tale of the Children of Hurin — all but the first posthumously assembled by CJRT. But The Lord of the Rings itself is perfectly fine and probably best to quote from, provided you can find a suitable quote there. 👑

So if you were to supply a new, complete, top-level source file to add to Perl, you should conform to this peculiar practice by yourself selecting an appropriate quotation from Tolkien, retaining the original spelling and punctuation and using the same format the rest of the quotes are in. Indirect and oblique is just fine; remember, it's a metaphor, so being meta is, after all, what it's for. 💅✨

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💖 AUTHOR 💖

This document was originally written by Nathan Torkington, and is maintained by the perl5-porters mailing list.

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okay that's the whole page, you made it to the end bestie, we're so proud of you 🥹 this was perl version v5.42.3, dated 2026-08-03, and it's the PERLHACK(1) man page — go forth and hack, the road goes ever on and on 💜🦋🎀