PERLNEWMOD(1) Perl Programmers Reference Guide PERLNEWMOD(1)
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 avail-
able 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 docu-
ment 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 pack-
aging up the solution into a module and uploading it to CPAN so that
others can benefit.
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 popu-
larity of the library you are gluing, the portability to other operat-
ing 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 for-
mat 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 cer-
tainly 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 modu-
larise. 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.
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
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 <http://search.cpan.org> 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 send-
ing out feelers on the "comp.lang.perl.modules" newsgroup, or as a
last resort, ask the modules list at "firstname.lastname@example.org". 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 suc-
While you’re doing that, make really sure you haven’t missed a mod-
ule similar to the one you’re about to write.
When you’ve got your name sorted out and you’re sure that your mod-
ule is wanted and not currently available, it’s time to start cod-
Step-by-step: Making the module
Start with module-starter or h2xs
The module-starter utility is distributed as part of the Mod-
ule::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
module-starter --module=Foo::Bar \
--author="Your Name" --email@example.com
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
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/Acme.pm
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 mod-
ule at fault.
Use Exporter - wisely!
Exporter gives you a standard way of exporting symbols and subrou-
tines from your module into the caller’s namespace. For instance,
saying "use Net::Acme qw(&frob)" would import the "frob" subroutine.
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 intro-
duction. 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.
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, "mod-
ule-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
"http://pause.perl.org/", 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 down-
load and install modules, and this produces a Makefile with a "dist"
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"
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, "firstname.lastname@example.org"
Updated by Kirrily "Skud" Robert, "email@example.com"
perlmod, perlmodlib, perlmodinstall, h2xs, strict, Carp, Exporter,
perlpod, Test::Simple, Test::More ExtUtils::MakeMaker, Module::Build,
Module::Starter http://www.cpan.org/ , Ken Williams’ tutorial on build-
ing your own module at http://mathforum.org/~ken/perl_modules.html
perl v5.8.6 2004-11-05 PERLNEWMOD(1)
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