PERLMODSTYLE(1) Perl Programmers Reference Guide PERLMODSTYLE(1)
perlmodstyle - Perl module style guide
This document attempts to describe the Perl Community's "best practice"
for writing Perl modules. It extends the recommendations found in
perlstyle , which should be considered required reading before reading
While this document is intended to be useful to all module authors, it
is particularly aimed at authors who wish to publish their modules on
The focus is on elements of style which are visible to the users of a
module, rather than those parts which are only seen by the module's
developers. However, many of the guidelines presented in this document
can be extrapolated and applied successfully to a module's internals.
This document differs from perlnewmod in that it is a style guide
rather than a tutorial on creating CPAN modules. It provides a check-
list against which modules can be compared to determine whether they
conform to best practice, without necessarily describing in detail how
to achieve this.
All the advice contained in this document has been gleaned from exten-
sive conversations with experienced CPAN authors and users. Every
piece of advice given here is the result of previous mistakes. This
information is here to help you avoid the same mistakes and the extra
work that would inevitably be required to fix them.
The first section of this document provides an itemized checklist; sub-
sequent sections provide a more detailed discussion of the items on the
list. The final section, "Common Pitfalls", describes some of the most
popular mistakes made by CPAN authors.
For more detail on each item in this checklist, see below.
Before you start
o Don't re-invent the wheel
o Patch, extend or subclass an existing module where possible
o Do one thing and do it well
o Choose an appropriate name
o API should be understandable by the average programmer
o Simple methods for simple tasks
o Separate functionality from output
o Consistent naming of subroutines or methods
o Use named parameters (a hash or hashref) when there are more than
o Ensure your module works under "use strict" and "-w"
o Stable modules should maintain backwards compatibility
o Write documentation in POD
o Document purpose, scope and target applications
o Document each publically accessible method or subroutine, including
params and return values
o Give examples of use in your documentation
o Provide a README file and perhaps also release notes, changelog,
o Provide links to further information (URL, email)
o Specify pre-requisites in Makefile.PL or Build.PL
o Specify Perl version requirements with "use"
o Include tests with your module
o Choose a sensible and consistent version numbering scheme (X.YY is
the common Perl module numbering scheme)
o Increment the version number for every change, no matter how small
o Package the module using "make dist"
o Choose an appropriate license (GPL/Artistic is a good default)
BEFORE YOU START WRITING A MODULE
Try not to launch headlong into developing your module without spending
some time thinking first. A little forethought may save you a vast
amount of effort later on.
Has it been done before?
You may not even need to write the module. Check whether it's already
been done in Perl, and avoid re-inventing the wheel unless you have a
Good places to look for pre-existing modules include
http://search.cpan.org/ and asking on firstname.lastname@example.org
If an existing module almost does what you want, consider writing a
patch, writing a subclass, or otherwise extending the existing module
rather than rewriting it.
Do one thing and do it well
At the risk of stating the obvious, modules are intended to be modular.
A Perl developer should be able to use modules to put together the
building blocks of their application. However, it's important that the
blocks are the right shape, and that the developer shouldn't have to
use a big block when all they need is a small one.
Your module should have a clearly defined scope which is no longer than
a single sentence. Can your module be broken down into a family of
"FooBar.pm provides an implementation of the FOO protocol and the
related BAR standard."
"Foo.pm provides an implementation of the FOO protocol. Bar.pm imple-
ments the related BAR protocol."
This means that if a developer only needs a module for the BAR stan-
dard, they should not be forced to install libraries for FOO as well.
What's in a name?
Make sure you choose an appropriate name for your module early on.
This will help people find and remember your module, and make program-
ming with your module more intuitive.
When naming your module, consider the following:
o Be descriptive (i.e. accurately describes the purpose of the mod-
o Be consistent with existing modules.
o Reflect the functionality of the module, not the implementation.
o Avoid starting a new top-level hierarchy, especially if a suitable
hierarchy already exists under which you could place your module.
You should contact email@example.com to ask them about your module name
before publishing your module. You should also try to ask people who
are already familiar with the module's application domain and the CPAN
naming system. Authors of similar modules, or modules with similar
names, may be a good place to start.
DESIGNING AND WRITING YOUR MODULE
Considerations for module design and coding:
To OO or not to OO?
Your module may be object oriented (OO) or not, or it may have both
kinds of interfaces available. There are pros and cons of each tech-
nique, which should be considered when you design your API.
According to Damian Conway, you should consider using OO:
o When the system is large or likely to become so
o When the data is aggregated in obvious structures that will become
o When the types of data form a natural hierarchy that can make use
o When operations on data vary according to data type (making poly-
morphic invocation of methods feasible)
o When it is likely that new data types may be later introduced into
the system, and will need to be handled by existing code
o When interactions between data are best represented by overloaded
o When the implementation of system components is likely to change
over time (and hence should be encapsulated)
o When the system design is itself object-oriented
o When large amounts of client code will use the software (and should
be insulated from changes in its implementation)
o When many separate operations will need to be applied to the same
set of data
Think carefully about whether OO is appropriate for your module. Gra-
tuitous object orientation results in complex APIs which are difficult
for the average module user to understand or use.
Designing your API
Your interfaces should be understandable by an average Perl programmer.
The following guidelines may help you judge whether your API is suffi-
Write simple routines to do simple things.
It's better to have numerous simple routines than a few monolithic
ones. If your routine changes its behaviour significantly based on
its arguments, it's a sign that you should have two (or more) sepa-
Separate functionality from output.
Return your results in the most generic form possible and allow the
user to choose how to use them. The most generic form possible is
usually a Perl data structure which can then be used to generate a
text report, HTML, XML, a database query, or whatever else your
If your routine iterates through some kind of list (such as a list
of files, or records in a database) you may consider providing a
callback so that users can manipulate each element of the list in
turn. File::Find provides an example of this with its
"find(\&wanted, $dir)" syntax.
Provide sensible shortcuts and defaults.
Don't require every module user to jump through the same hoops to
achieve a simple result. You can always include optional parame-
ters or routines for more complex or non-standard behaviour. If
most of your users have to type a few almost identical lines of
code when they start using your module, it's a sign that you should
have made that behaviour a default. Another good indicator that
you should use defaults is if most of your users call your routines
with the same arguments.
Your naming should be consistent. For instance, it's better to
This applies equally to method names, parameter names, and anything
else which is visible to the user (and most things that aren't!)
Use named parameters. It's easier to use a hash like this:
name => "wibble",
type => "text",
size => 1024,
... than to have a long list of unnamed parameters like this:
$obj->do_something("wibble", "text", 1024);
While the list of arguments might work fine for one, two or even
three arguments, any more arguments become hard for the module user
to remember, and hard for the module author to manage. If you want
to add a new parameter you will have to add it to the end of the
list for backward compatibility, and this will probably make your
list order unintuitive. Also, if many elements may be undefined
you may see the following unattractive method calls:
$obj->do_something(undef, undef, undef, undef, undef, undef, 1024);
Provide sensible defaults for parameters which have them. Don't
make your users specify parameters which will almost always be the
The issue of whether to pass the arguments in a hash or a hashref
is largely a matter of personal style.
The use of hash keys starting with a hyphen ("-name") or entirely
in upper case ("NAME") is a relic of older versions of Perl in
which ordinary lower case strings were not handled correctly by the
"=>" operator. While some modules retain uppercase or hyphenated
argument keys for historical reasons or as a matter of personal
style, most new modules should use simple lower case keys. What-
ever you choose, be consistent!
Strictness and warnings
Your module should run successfully under the strict pragma and should
run without generating any warnings. Your module should also handle
taint-checking where appropriate, though this can cause difficulties in
Modules which are "stable" should not break backwards compatibility
without at least a long transition phase and a major change in version
Error handling and messages
When your module encounters an error it should do one or more of:
o Return an undefined value.
o set $Module::errstr or similar ("errstr" is a common name used by
DBI and other popular modules; if you choose something else, be
sure to document it clearly).
o "warn()" or "carp()" a message to STDERR.
o "croak()" only when your module absolutely cannot figure out what
to do. ("croak()" is a better version of "die()" for use within
modules, which reports its errors from the perspective of the
caller. See Carp for details of "croak()", "carp()" and other use-
o As an alternative to the above, you may prefer to throw exceptions
using the Error module.
Configurable error handling can be very useful to your users. Consider
offering a choice of levels for warning and debug messages, an option
to send messages to a separate file, a way to specify an error-handling
routine, or other such features. Be sure to default all these options
to the commonest use.
DOCUMENTING YOUR MODULE
Your module should include documentation aimed at Perl developers. You
should use Perl's "plain old documentation" (POD) for your general
technical documentation, though you may wish to write additional docu-
mentation (white papers, tutorials, etc) in some other format. You
need to cover the following subjects:
o A synopsis of the common uses of the module
o The purpose, scope and target applications of your module
o Use of each publically accessible method or subroutine, including
parameters and return values
o Examples of use
o Sources of further information
o A contact email address for the author/maintainer
The level of detail in Perl module documentation generally goes from
less detailed to more detailed. Your SYNOPSIS section should contain a
minimal example of use (perhaps as little as one line of code; skip the
unusual use cases or anything not needed by most users); the DESCRIP-
TION should describe your module in broad terms, generally in just a
few paragraphs; more detail of the module's routines or methods,
lengthy code examples, or other in-depth material should be given in
Ideally, someone who's slightly familiar with your module should be
able to refresh their memory without hitting "page down". As your
reader continues through the document, they should receive a progres-
sively greater amount of knowledge.
The recommended order of sections in Perl module documentation is:
o One or more sections or subsections giving greater detail of avail-
able methods and routines and any other relevant information.
o SEE ALSO
o COPYRIGHT and LICENSE
Keep your documentation near the code it documents ("inline" documenta-
tion). Include POD for a given method right above that method's sub-
routine. This makes it easier to keep the documentation up to date,
and avoids having to document each piece of code twice (once in POD and
once in comments).
README, INSTALL, release notes, changelogs
Your module should also include a README file describing the module and
giving pointers to further information (website, author email).
An INSTALL file should be included, and should contain simple installa-
tion instructions. When using ExtUtils::MakeMaker this will usually be:
When using Module::Build, this will usually be:
perl Build test
perl Build install
Release notes or changelogs should be produced for each release of your
software describing user-visible changes to your module, in terms rele-
vant to the user.
Version numbers should indicate at least major and minor releases, and
possibly sub-minor releases. A major release is one in which most of
the functionality has changed, or in which major new functionality is
added. A minor release is one in which a small amount of functionality
has been added or changed. Sub-minor version numbers are usually used
for changes which do not affect functionality, such as documentation
The most common CPAN version numbering scheme looks like this:
1.00, 1.10, 1.11, 1.20, 1.30, 1.31, 1.32
A correct CPAN version number is a floating point number with at least
2 digits after the decimal. You can test whether it conforms to CPAN by
perl -MExtUtils::MakeMaker -le 'print MM->parse_version(shift)' 'Foo.pm'
If you want to release a 'beta' or 'alpha' version of a module but
don't want CPAN.pm to list it as most recent use an '_' after the regu-
lar version number followed by at least 2 digits, eg. 1.20_01. If you
do this, the following idiom is recommended:
$VERSION = "1.12_01";
$XS_VERSION = $VERSION; # only needed if you have XS code
$VERSION = eval $VERSION;
With that trick MakeMaker will only read the first line and thus read
the underscore, while the perl interpreter will evaluate the $VERSION
and convert the string into a number. Later operations that treat $VER-
SION as a number will then be able to do so without provoking a warning
about $VERSION not being a number.
Never release anything (even a one-word documentation patch) without
incrementing the number. Even a one-word documentation patch should
result in a change in version at the sub-minor level.
Module authors should carefully consider whether to rely on other mod-
ules, and which modules to rely on.
Most importantly, choose modules which are as stable as possible. In
order of preference:
o Core Perl modules
o Stable CPAN modules
o Unstable CPAN modules
o Modules not available from CPAN
Specify version requirements for other Perl modules in the pre-requi-
sites in your Makefile.PL or Build.PL.
Be sure to specify Perl version requirements both in Makefile.PL or
Build.PL and with "require 5.6.1" or similar. See the section on "use
VERSION" of "require" in perlfunc for details.
All modules should be tested before distribution (using "make
disttest"), and the tests should also be available to people installing
the modules (using "make test"). For Module::Build you would use the
"make test" equivalent "perl Build test".
The importance of these tests is proportional to the alleged stability
of a module -- a module which purports to be stable or which hopes to
achieve wide use should adhere to as strict a testing regime as possi-
Useful modules to help you write tests (with minimum impact on your
development process or your time) include Test::Simple, Carp::Assert
and Test::Inline. For more sophisticated test suites there are
Test::More and Test::MockObject.
Modules should be packaged using one of the standard packaging tools.
Currently you have the choice between ExtUtils::MakeMaker and the more
platform independent Module::Build, allowing modules to be installed in
a consistent manner. When using ExtUtils::MakeMaker, you can use "make
dist" to create your package. Tools exist to help you to build your
module in a MakeMaker-friendly style. These include ExtUtils::Module-
Maker and h2xs. See also perlnewmod.
Make sure that your module has a license, and that the full text of it
is included in the distribution (unless it's a common one and the terms
of the license don't require you to include it).
If you don't know what license to use, dual licensing under the GPL and
Artistic licenses (the same as Perl itself) is a good idea. See perl-
gpl and perlartistic.
Reinventing the wheel
There are certain application spaces which are already very, very well
served by CPAN. One example is templating systems, another is date and
time modules, and there are many more. While it is a rite of passage
to write your own version of these things, please consider carefully
whether the Perl world really needs you to publish it.
Trying to do too much
Your module will be part of a developer's toolkit. It will not, in
itself, form the entire toolkit. It's tempting to add extra features
until your code is a monolithic system rather than a set of modular
Don't fall into the trap of writing for the wrong audience. Your pri-
mary audience is a reasonably experienced developer with at least a
moderate understanding of your module's application domain, who's just
downloaded your module and wants to start using it as quickly as possi-
Tutorials, end-user documentation, research papers, FAQs etc are not
appropriate in a module's main documentation. If you really want to
write these, include them as sub-documents such as "My::Module::Tuto-
rial" or "My::Module::FAQ" and provide a link in the SEE ALSO section
of the main documentation.
General Perl style guide
How to create a new module
Verifies your POD's correctness
Test::Simple, Test::Inline, Carp::Assert, Test::More, Test::MockOb-
Perl Authors Upload Server. Contains links to information for mod-
Any good book on software engineering
Kirrily "Skud" Robert
perl v5.8.8 2006-01-07 PERLMODSTYLE(1)