perlnewmod - preparing a new module for distribution


   This document gives you some suggestions about how to go about writing
   Perl modules, preparing them for distribution, and making them
   available via CPAN.

   One of the things that makes Perl really powerful is the fact that Perl
   hackers tend to want to share the solutions to problems they've faced,
   so you and I don't have to battle with the same problem again.

   The main way they do this is by abstracting the solution into a Perl
   module. If you don't know what one of these is, the rest of this
   document isn't going to be much use to you. You're also missing out on
   an awful lot of useful code; consider having a look at perlmod,
   perlmodlib and perlmodinstall before coming back here.

   When you've found that there isn't a module available for what you're
   trying to do, and you've had to write the code yourself, consider
   packaging up the solution into a module and uploading it to CPAN so
   that others can benefit.

   You should also take a look at perlmodstyle for best practices in
   making a module.

   We're going to primarily concentrate on Perl-only modules here, rather
   than XS modules. XS modules serve a rather different purpose, and you
   should consider different things before distributing them - the
   popularity of the library you are gluing, the portability to other
   operating systems, and so on. However, the notes on preparing the Perl
   side of the module and packaging and distributing it will apply equally
   well to an XS module as a pure-Perl one.

   What should I make into a module?
   You should make a module out of any code that you think is going to be
   useful to others. Anything that's likely to fill a hole in the communal
   library and which someone else can slot directly into their program.
   Any part of your code which you can isolate and extract and plug into
   something else is a likely candidate.

   Let's take an example. Suppose you're reading in data from a local
   format into a hash-of-hashes in Perl, turning that into a tree, walking
   the tree and then piping each node to an Acme Transmogrifier Server.

   Now, quite a few people have the Acme Transmogrifier, and you've had to
   write something to talk the protocol from scratch - you'd almost
   certainly want to make that into a module. The level at which you pitch
   it is up to you: you might want protocol-level modules analogous to
   Net::SMTP which then talk to higher level modules analogous to
   Mail::Send. The choice is yours, but you do want to get a module out
   for that server protocol.

   Nobody else on the planet is going to talk your local data format, so
   we can ignore that. But what about the thing in the middle? Building
   tree structures from Perl variables and then traversing them is a nice,
   general problem, and if nobody's already written a module that does
   that, you might want to modularise that code too.

   So hopefully you've now got a few ideas about what's good to
   modularise.  Let's now see how it's done.

   Step-by-step: Preparing the ground
   Before we even start scraping out the code, there are a few things
   we'll want to do in advance.

   Look around
      Dig into a bunch of modules to see how they're written. I'd suggest
      starting with Text::Tabs, since it's in the standard library and is
      nice and simple, and then looking at something a little more complex
      like File::Copy.  For object oriented code, "WWW::Mechanize" or the
      "Email::*" modules provide some good examples.

      These should give you an overall feel for how modules are laid out
      and written.

   Check it's new
      There are a lot of modules on CPAN, and it's easy to miss one that's
      similar to what you're planning on contributing. Have a good plough
      through the <> and make sure you're not the
      one reinventing the wheel!

   Discuss the need
      You might love it. You might feel that everyone else needs it. But
      there might not actually be any real demand for it out there. If
      you're unsure about the demand your module will have, consider
      sending out feelers on the "comp.lang.perl.modules" newsgroup, or as
      a last resort, ask the modules list at "". Remember
      that this is a closed list with a very long turn-around time - be
      prepared to wait a good while for a response from them.

   Choose a name
      Perl modules included on CPAN have a naming hierarchy you should try
      to fit in with. See perlmodlib for more details on how this works,
      and browse around CPAN and the modules list to get a feel of it. At
      the very least, remember this: modules should be title capitalised,
      (This::Thing) fit in with a category, and explain their purpose

   Check again
      While you're doing that, make really sure you haven't missed a
      module similar to the one you're about to write.

      When you've got your name sorted out and you're sure that your
      module is wanted and not currently available, it's time to start

   Step-by-step: Making the module
   Start with module-starter or h2xs
      The module-starter utility is distributed as part of the
      Module::Starter CPAN package.  It creates a directory with stubs of
      all the necessary files to start a new module, according to recent
      "best practice" for module development, and is invoked from the
      command line, thus:

          module-starter --module=Foo::Bar \
             --author="Your Name"

      If you do not wish to install the Module::Starter package from CPAN,
      h2xs is an older tool, originally intended for the development of XS
      modules, which comes packaged with the Perl distribution.

      A typical invocation of h2xs for a pure Perl module is:

          h2xs -AX --skip-exporter --use-new-tests -n Foo::Bar

      The "-A" omits the Autoloader code, "-X" omits XS elements,
      "--skip-exporter" omits the Exporter code, "--use-new-tests" sets up
      a modern testing environment, and "-n" specifies the name of the

   Use strict and warnings
      A module's code has to be warning and strict-clean, since you can't
      guarantee the conditions that it'll be used under. Besides, you
      wouldn't want to distribute code that wasn't warning or strict-clean
      anyway, right?

   Use Carp
      The Carp module allows you to present your error messages from the
      caller's perspective; this gives you a way to signal a problem with
      the caller and not your module. For instance, if you say this:

          warn "No hostname given";

      the user will see something like this:

       No hostname given at
       /usr/local/lib/perl5/site_perl/5.6.0/Net/ line 123.

      which looks like your module is doing something wrong. Instead, you
      want to put the blame on the user, and say this:

          No hostname given at bad_code, line 10.

      You do this by using Carp and replacing your "warn"s with "carp"s.
      If you need to "die", say "croak" instead. However, keep "warn" and
      "die" in place for your sanity checks - where it really is your
      module at fault.

   Use Exporter - wisely!
      Exporter gives you a standard way of exporting symbols and
      subroutines from your module into the caller's namespace. For
      instance, saying "use Net::Acme qw(&frob)" would import the "frob"

      The package variable @EXPORT will determine which symbols will get
      exported when the caller simply says "use Net::Acme" - you will
      hardly ever want to put anything in there. @EXPORT_OK, on the other
      hand, specifies which symbols you're willing to export. If you do
      want to export a bunch of symbols, use the %EXPORT_TAGS and define a
      standard export set - look at Exporter for more details.

   Use plain old documentation
      The work isn't over until the paperwork is done, and you're going to
      need to put in some time writing some documentation for your module.
      "module-starter" or "h2xs" will provide a stub for you to fill in;
      if you're not sure about the format, look at perlpod for an
      introduction. Provide a good synopsis of how your module is used in
      code, a description, and then notes on the syntax and function of
      the individual subroutines or methods. Use Perl comments for
      developer notes and POD for end-user notes.

   Write tests
      You're encouraged to create self-tests for your module to ensure
      it's working as intended on the myriad platforms Perl supports; if
      you upload your module to CPAN, a host of testers will build your
      module and send you the results of the tests. Again,
      "module-starter" and "h2xs" provide a test framework which you can
      extend - you should do something more than just checking your module
      will compile.  Test::Simple and Test::More are good places to start
      when writing a test suite.

   Write the README
      If you're uploading to CPAN, the automated gremlins will extract the
      README file and place that in your CPAN directory. It'll also appear
      in the main by-module and by-category directories if you make it
      onto the modules list. It's a good idea to put here what the module
      actually does in detail, and the user-visible changes since the last

   Step-by-step: Distributing your module
   Get a CPAN user ID
      Every developer publishing modules on CPAN needs a CPAN ID.  Visit
      "", select "Request PAUSE Account", and wait
      for your request to be approved by the PAUSE administrators.

   "perl Makefile.PL; make test; make dist"
      Once again, "module-starter" or "h2xs" has done all the work for
      you.  They produce the standard "Makefile.PL" you see when you
      download and install modules, and this produces a Makefile with a
      "dist" target.

      Once you've ensured that your module passes its own tests - always a
      good thing to make sure - you can "make dist", and the Makefile will
      hopefully produce you a nice tarball of your module, ready for

   Upload the tarball
      The email you got when you received your CPAN ID will tell you how
      to log in to PAUSE, the Perl Authors Upload SErver. From the menus
      there, you can upload your module to CPAN.

   Announce to the modules list
      Once uploaded, it'll sit unnoticed in your author directory. If you
      want it connected to the rest of the CPAN, you'll need to go to
      "Register Namespace" on PAUSE.  Once registered, your module will
      appear in the by-module and by-category listings on CPAN.

   Announce to clpa
      If you have a burning desire to tell the world about your release,
      post an announcement to the moderated "comp.lang.perl.announce"

   Fix bugs!
      Once you start accumulating users, they'll send you bug reports. If
      you're lucky, they'll even send you patches. Welcome to the joys of
      maintaining a software project...


   Simon Cozens, ""

   Updated by Kirrily "Skud" Robert, ""


   perlmod, perlmodlib, perlmodinstall, h2xs, strict, Carp, Exporter,
   perlpod, Test::Simple, Test::More ExtUtils::MakeMaker, Module::Build,
   Module::Starter , Ken Williams's tutorial on
   building your own module at

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