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perlhack - How to hack on Perl


This document explains how Perl development works. It includes details about the Perl 5 Porters email list, the Perl repository, the Perl bug tracker, patch guidelines, and commentary on Perl development philosophy.


If you just want to submit a single small patch like a pod fix, a test for a bug, comment fixes, etc., it's easy! Here's how:


If you want to report a bug in Perl, or browse existing Perl bugs and patches, use the GitHub issue tracker at

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 that the bug has been reported already.

You can log in to the bug tracking system and comment on existing bug reports. If you have additional information regarding an existing bug, please add it. This will help the porters fix the bug.


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

A searchable archive of the list is available at There is also an archive at

perl-changes mailing list

The perl5-changes mailing list receives a copy of each patch that gets submitted to the maintenance and development branches of the perl repository. See for subscription and archive information.

#p5p on IRC

Many porters are also active on the irc:// channel. Feel free to join the channel and ask questions about hacking on the Perl core.


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

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

Read access via Git

You will need a copy of Git for your computer. You can fetch a copy of the repository using the git protocol:

% git clone perl

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

If you cannot use the git protocol for firewall reasons, you can also clone via http:

% git clone perl

Read access via the web

You may access the repository over the web. This allows you to browse the tree, see recent commits, subscribe to repository notifications, search for particular commits and more. You may access it at

Write access via git

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


If you're planning to do more extensive work than a single small fix, we encourage you to read the documentation below. This will 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 will receive email.

In other cases, the patch will need more work or discussion. You are 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 that 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 are submitting a code patch there are several things that 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 that blead is up to date before producing the diff, that's why we call git pull first.

We strongly recommend that you use git if possible. It will 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 that 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 being 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.

A commit message isn't intended to take the place of comments in your code. Commit messages should describe the change you made, while code comments should describe the current state of the code.

If you've just implemented a new feature, complete with doc, tests and well-commented code, a brief commit message will often suffice. If, however, you've just changed a single character deep in the parser or lexer, you might need to write a small novel to ensure that future readers understand what you did and why you did it.

Comments, Comments, Comments

Be sure to adequately comment your code. While commenting every line is unnecessary, anything that takes advantage of side effects of operators, that creates changes that will be felt outside of the function being patched, or that others may find confusing should be documented. If you are going to err, it is better to err on the side of adding too many comments than too few.

The best comments explain why the code does what it does, not what it does.


In general, please follow the particular style of the code you are patching.

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

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 <>):

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 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/ file to flag that a local modification has been made. See Porting/ 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:

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?

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.

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.

Does it create too much work?

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

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.


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.

There are three ways to write a test in the core: Test::More, t/ 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 breaking) from causing basic functionality tests to fail.

The t/ library provides some of the features of Test::More, but avoids loading most modules and uses as few core features as possible.

If you write your own test, use the Test Anything Protocol.

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/ 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.

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:

If you are testing locales (see perllocale), there are helper functions in t/ 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.

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.

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

You can run part of the test suite by hand by using one of the following commands from the t/ directory:

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


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

(If you don't specify test scripts, the whole test suite will be run.)

Using t/harness for testing

If you use harness for testing, you have several command line options available to you. The arguments are as follows, and are in the order that they must appear if used together.

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 omitted, the file list is obtained from the manifest. The file list may include shell wildcards which will be expanded out.

You can run an individual test by a command similar to

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

except that the harnesses set up some environment variables that may affect the execution of the test:

Other environment variables that may influence tests

See also the documentation for the Test and Test::Harness modules, for more environment variables that affect testing.

Performance testing

The file t/perf/benchmarks contains snippets of perl code which are intended to be benchmarked across a range of perls by the Porting/ tool. If you fix or enhance a performance issue, you may want to add a representative code sample to the file, then run against the previous and current perls to see what difference it has made, and whether anything else has slowed down as a consequence.

The file t/perf/opcount.t is designed to test whether a particular code snippet has been compiled into an optree containing specified numbers of particular op types. This is good for testing whether optimisations which alter ops, such as converting an aelem op into an aelemfast op, are really doing that.

The files t/perf/speed.t and t/re/speed.t are designed 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). Add a test that will take a fraction of a second normally, and minutes otherwise, causing the test file to time out on failure.

Building perl at older commits

In the course of hacking on the Perl core distribution, you may have occasion to configure, build and test perl at an old commit. Sometimes make will fail during this 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. Use of Porting/ had identified commit ba77e4cc9d1ceebf472c9c5c18b2377ee47062e6 as the commit in which a bug was 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 installation and use of Devel::PatchPerl for this situation, first to determine the version of perl at the commit in question, then to patch the source code at that point to facilitate a build.

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


To hack on the Perl guts, you'll need to read the following things:


The CPAN testers ( ) are a group of volunteers who test CPAN modules on a variety of platforms.

Perl Smokers ( and ) automatically test Perl source releases on platforms with various configurations.

Both efforts welcome volunteers. In order to get involved in smoke testing of the perl itself visit In order to start smoke testing CPAN modules visit or or


If you've read all the documentation in the document and the ones listed above, you're more than ready to hack on Perl.

Here's some more recommendations

"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.

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.

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:

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.


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