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PERLHACK(1pm)          Perl Programmers Reference Guide          PERLHACK(1pm)


       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


       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:

       o   Check out the source repository

           The perl source is in a git repository.  You can clone the
           repository with the following command:

             % git clone perl

       o   Ensure you're following the latest advice

           In case the advice in this guide has been updated recently, read
           the latest version directly from the perl source:

             % perldoc pod/perlhack.pod

       o   Create a branch for your change

           Create a branch based on blead to commit your change to, which will
           later be used to send it to the Perl issue tracker.

             % git checkout -b mychange

       o   Make your change

           Hack, hack, hack.  Keep in mind that Perl runs on many different
           platforms, with different operating systems that have different
           capabilities, different filesystem organizations, and even
           different character sets.  perlhacktips gives advice on this.

       o   Test your change

           You can run all the tests with the following commands:

             % ./Configure -des -Dusedevel
             % make test

           Keep hacking until the tests pass.

       o   Commit your change

           Committing your work will save 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".

       o   Send your change to the Perl issue tracker

           The next step is to submit your patch to the Perl core ticket

           Create a GitHub fork of the perl5 repository and add it as a
           remote, if you haven't already, as described in the GitHub
           documentation at

             % git remote add fork

           For more information, see "Connecting to GitHub with SSH"

           If you'd rather use an HTTPS URL for your "git push" see "Cloning
           with HTTPS URLs" <

             % git remote add fork

           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
           as described in the GitHub documentation at

       o   Thank you

           The porters appreciate the time you spent helping to make Perl
           better.  Thank you!

       o   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 donXt want to be included in the AUTHORS file, just let us
           know. Otherwise we will take your submission of a patch as
           permission to credit you in the AUTHORS file.

       o   Next time

           The next time you wish to make a patch, you need to 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 which you wish to
           keep, then run these commands:

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


       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

       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

   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 git:// perl

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

       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

   Read access via rsync
       You can also choose to use rsync to get a copy of the current source
       tree for the bleadperl branch and all maintenance branches:

         % rsync -avz rsync:// .
         % rsync -avz rsync:// .
         % rsync -avz rsync:// .
         % rsync -avz rsync:// .
         % rsync -avz rsync:// .
         % rsync -avz rsync:// .

       (Add the "--delete" option to remove leftover files.)

       To get a full list of the available sync points:

         % rsync

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


       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.

       o   Why

           Your commit message should describe why the change you are 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.

       o   What

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

       o   How

           While it's not necessary for documentation changes, new tests or
           trivial patches, it's often worth explaining how your change works.
           Even if it's clear to you today, it may not be clear to a porter
           next month or next year.

       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


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

       In particular, follow these general guidelines for patching Perl

       o   4-wide indents for code, 2-wide indents for nested CPP "#define"s,
           with 8-wide tabstops.

       o   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.

       o   Try hard not to exceed 79-columns

       o   ANSI C prototypes

       o   Uncuddled elses and "K&R" style for indenting control constructs

       o   No C++ style (//) comments

       o   Mark places that need to be revisited with XXX (and revisit often!)

       o   Opening brace lines up with "if" when conditional spans multiple
           lines; should be at end-of-line otherwise

       o   In function definitions, name starts in column 0 (return value-type
           is on previous line)

       o   Single space after keywords that are followed by parens, no space
           between function name and following paren

       o   Avoid assignments in conditionals, but if they're unavoidable, use
           extra paren, e.g. "if (a && (b = c)) ..."

       o   "return foo;" rather than "return(foo);"

       o   "if (!foo) ..." rather than "if (foo == FALSE) ..." etc.

       o   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.

       o   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

       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 <>):

       o   Know what you're testing.  Read the docs, and the source.

       o   Tend to fail, not succeed.

       o   Interpret results strictly.

       o   Use unrelated features (this will flush out bizarre interactions).

       o   Use non-standard idioms (otherwise you are not testing TIMTOWTDI).

       o   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).

       o   Give meaningful error messages when a test fails.

       o   Avoid using qx// and system() unless you are testing for them.  If
           you do use them, make sure that you cover _all_ perl platforms.

       o   Unlink any temporary files you create.

       o   Promote unforeseen warnings to errors with $SIG{__WARN__}.

       o   Be sure to use the libraries and modules shipped with the version
           being tested, not those that were already installed.

       o   Add comments to the code explaining what you are testing for.

       o   Make updating the '1..42' string unnecessary.  Or make sure that
           you update it.

       o   Test _all_ behaviors of a given operator, library, or function.

           Test all optional arguments.

           Test return values in various contexts (boolean, scalar, list,

           Use both global and lexical variables.

           Don't forget the exceptional, pathological cases.

   Patching a core module
       This works just like patching anything else, with one extra

       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:

       o   Adding, deprecating, or removing core features

       o   Adding, deprecating, removing, or upgrading core or dual-life

       o   Adding new core tests

       o   Fixing security issues and user-visible bugs in the core

       o   Changes that might break existing code, either on the perl or C

       o   Significant performance improvements

       o   Adding, removing, or significantly changing documentation in the
           pod/ directory

       o   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

   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

       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

       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

       o   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/ presumes has already been demonstrated to work.

       o   All other subdirectories of t/

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

           You can also use certain libraries like Config conditionally, but
           be sure to skip the test gracefully if it's not there.

       o   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/ 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:

       o   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".

       o   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/ 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

       o   test_porting

           This runs some basic sanity tests on the source tree and helps
           catch basic errors before you submit a patch.

       o   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, utilties, 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.

       o   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

       o   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 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"

       o   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.

       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

   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.

       o   -v

           Run the tests under verbose mode so you can see what tests were
           run, and debug output.

       o   -torture

           Run the torture tests as well as the normal set.

       o   -re=PATTERN

           Filter the file list so that all the test files run match PATTERN.
           Note that this form is distinct from the -re LIST OF PATTERNS form
           below in that it allows the file list to be provided as well.

       o   -re LIST OF PATTERNS

           Filter the file list so that all the test files run match
           /(LIST|OF|PATTERNS)/.  Note that with this form the patterns are
           joined by '|' and you cannot supply a list of files, instead the
           test files are obtained from the MANIFEST.

       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:

       o   PERL_CORE=1

           indicates that we're running this test as part of the perl core
           test suite.  This is useful for modules that have a dual life on


           is set to 2 if it isn't set already (see "PERL_DESTRUCT_LEVEL" in

       o   PERL

           (used only by t/TEST) if set, overrides the path to the perl
           executable that should be used to run the tests (the default being


           if set, tells to skip the tests that need a terminal.  It's
           actually set automatically by the Makefile, but can also be forced
           artificially by running 'make test_notty'.

       Other environment variables that may influence tests

       o   PERL_TEST_Net_Ping

           Setting this variable runs all the Net::Ping modules tests,
           otherwise some tests that interact with the outside world are
           skipped.  See perl58delta.


           Setting this variable skips the vrexx.t tests for OS2::REXX.


           This sets a variable in op/numconvert.t.


           Setting this variable includes the tests in t/bigmem/.  This should
           be set to the number of gigabytes of memory available for testing,
           eg.  "PERL_TEST_MEMORY=4" indicates that tests that require 4GiB of
           available memory can be run safely.

       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

       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

           $ 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"), "\n";'
           $ 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:

       o   perlsource

           An overview of the Perl source tree.  This will help you find the
           files you're looking for.

       o   perlinterp

           An overview of the Perl interpreter source code and some details on
           how Perl does what it does.

       o   perlhacktut

           This document walks through the creation of 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.

       o   perlhacktips

           More details on hacking the Perl core.  This document focuses on
           lower level details such as how to write tests, compilation issues,
           portability, debugging, etc.

           If you plan on doing serious C hacking, make sure to read this.

       o   perlguts

           This is of paramount importance, since 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 in conjunction with
           poking at Perl source, and we'll do that later on.

           Gisle Aas's "illustrated perlguts", also known as illguts, has very
           helpful pictures:


       o   perlxstut and perlxs

           A working knowledge of XSUB programming is incredibly useful for
           core hacking; XSUBs use techniques drawn from the PP code, 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.

       o   perlapi

           The documentation for the Perl API explains what some of the
           internal functions do, as well as the many macros used in the

       o   Porting/pumpkin.pod

           This is 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.


       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

       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

       o   Subscribe to perl5-porters, follow the patches and try and
           understand them; don't be afraid to ask if there's a portion you're
           not clear on - who knows, you may unearth a bug in the patch...

       o   Do read the README associated with your operating system, e.g.
           README.aix on the IBM AIX OS.  Don't hesitate to supply patches to
           that README if you find anything missing or changed over a new OS

       o   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.

   "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

   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:

       o   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.

       o   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

       o   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.


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

perl v5.34.0                      2021-02-21                     PERLHACK(1pm)

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