Manual Reference Pages  - PERLMODLIB (1)


perlmodlib - constructing new Perl modules and finding existing ones



Many modules are included in the Perl distribution. These are described below, and all end in .pm. You may discover compiled library files (usually ending in .so) or small pieces of modules to be autoloaded (ending in .al); these were automatically generated by the installation process. You may also discover files in the library directory that end in either .pl or .ph. These are old libraries supplied so that old programs that use them still run. The .pl files will all eventually be converted into standard modules, and the .ph files made by h2ph will probably end up as extension modules made by h2xs. (Some .ph values may already be available through the POSIX, Errno, or Fcntl modules.) The pl2pm file in the distribution may help in your conversion, but it’s just a mechanical process and therefore far from bulletproof.

Pragmatic Modules

They work somewhat like compiler directives (pragmata) in that they tend to affect the compilation of your program, and thus will usually work well only when used within a use, or no. Most of these are lexically scoped, so an inner BLOCK may countermand them by saying:

    no integer;
    no strict ’refs’;
    no warnings;

which lasts until the end of that BLOCK.

Some pragmas are lexically scoped—typically those that affect the $^H hints variable. Others affect the current package instead, like use vars and use subs, which allow you to predeclare a variables or subroutines within a particular file rather than just a block. Such declarations are effective for the entire file for which they were declared. You cannot rescind them with no vars or no subs.

The following pragmas are defined (and have their own documentation).
attributes Get/set subroutine or variable attributes
attrs Set/get attributes of a subroutine (deprecated)
autouse Postpone load of modules until a function is used
base Establish IS-A relationship with base classes at compile time
bigint Transparent BigInteger support for Perl
bignum Transparent BigNumber support for Perl
bigrat Transparent BigNumber/BigRational support for Perl
blib Use MakeMaker’s uninstalled version of a package
bytes Force byte semantics rather than character semantics
charnames Define character names for \N{named} string literal escapes
constant Declare constants
diagnostics Produce verbose warning diagnostics
encoding Allows you to write your script in non-ascii or non-utf8
fields Compile-time class fields
filetest Control the filetest permission operators
if use a Perl module if a condition holds
integer Use integer arithmetic instead of floating point
less Request less of something from the compiler
lib Manipulate @INC at compile time
locale Use and avoid POSIX locales for built-in operations
open Set default PerlIO layers for input and output
ops Restrict unsafe operations when compiling
overload Package for overloading Perl operations
re Alter regular expression behaviour
sigtrap Enable simple signal handling
sort Control sort() behaviour
strict Restrict unsafe constructs
subs Predeclare sub names
threads Perl extension allowing use of interpreter based threads from perl
threads::shared Perl extension for sharing data structures between threads
utf8 Enable/disable UTF-8 (or UTF-EBCDIC) in source code
vars Predeclare global variable names (obsolete)
vmsish Control VMS-specific language features
warnings Control optional warnings
warnings::register Warnings import function

Standard Modules

Standard, bundled modules are all expected to behave in a well-defined manner with respect to namespace pollution because they use the Exporter module. See their own documentation for details.

It’s possible that not all modules listed below are installed on your system. For example, the GDBM_File module will not be installed if you don’t have the gdbm library.
AnyDBM_File Provide framework for multiple DBMs
Attribute::Handlers Simpler definition of attribute handlers
AutoLoader Load subroutines only on demand
AutoSplit Split a package for autoloading
B The Perl Compiler
B::Asmdata Autogenerated data about Perl ops, used to generate bytecode
B::Assembler Assemble Perl bytecode
B::Bblock Walk basic blocks
B::Bytecode Perl compiler’s bytecode backend
B::C Perl compiler’s C backend
B::CC Perl compiler’s optimized C translation backend
B::Concise Walk Perl syntax tree, printing concise info about ops
B::Debug Walk Perl syntax tree, printing debug info about ops
B::Deparse Perl compiler backend to produce perl code
B::Disassembler Disassemble Perl bytecode
B::Lint Perl lint
B::Showlex Show lexical variables used in functions or files
B::Stackobj Helper module for CC backend
B::Stash Show what stashes are loaded
B::Terse Walk Perl syntax tree, printing terse info about ops
B::Xref Generates cross reference reports for Perl programs
Benchmark Benchmark running times of Perl code
ByteLoader Load byte compiled perl code
CGI Simple Common Gateway Interface Class
CGI::Apache Backward compatibility module for
CGI::Carp CGI routines for writing to the HTTPD (or other) error log
CGI::Cookie Interface to Netscape Cookies
CGI::Fast CGI Interface for Fast CGI
CGI::Pretty Module to produce nicely formatted HTML code
CGI::Push Simple Interface to Server Push
CGI::Switch Backward compatibility module for defunct CGI::Switch
CGI::Util Internal utilities used by CGI module
CPAN Query, download and build perl modules from CPAN sites
CPAN::FirstTime Utility for CPAN::Config file Initialization
CPAN::Nox Wrapper around without using any XS module
CPAN::Version Utility functions to compare CPAN versions
Carp Warn of errors (from perspective of caller)
Carp::Heavy Heavy machinery, no user serviceable parts inside
Class::ISA Report the search path for a class’s ISA tree
Class::Struct Declare struct-like datatypes as Perl classes
Config Access Perl configuration information
Cwd Get pathname of current working directory
DB Programmatic interface to the Perl debugging API (draft, subject to
DBM_Filter Filter DBM keys/values
DB_File Perl5 access to Berkeley DB version 1.x
Data::Dumper Stringified perl data structures, suitable for both printing and eval
Devel::DProf A Perl code profiler
Devel::PPPort Perl/Pollution/Portability
Devel::Peek A data debugging tool for the XS programmer
Devel::SelfStubber Generate stubs for a SelfLoading module
Digest Modules that calculate message digests
Digest::MD5 Perl interface to the MD5 Algorithm
Digest::base Digest base class
Digest::file Calculate digests of files
DirHandle Supply object methods for directory handles
Dumpvalue Provides screen dump of Perl data.
DynaLoader Dynamically load C libraries into Perl code
Encode Character encodings
Encode::Alias Alias definitions to encodings
Encode::Byte Single Byte Encodings
Encode::CJKConstants Internally used by Encode::??::ISO_2022_*
Encode::CN China-based Chinese Encodings
Encode::CN::HZ Internally used by Encode::CN
Encode::Config Internally used by Encode
Encode::EBCDIC EBCDIC Encodings
Encode::Encoder Object Oriented Encoder
Encode::Encoding Encode Implementation Base Class
Encode::Guess Guesses encoding from data
Encode::JP Japanese Encodings
Encode::JP::H2Z Internally used by Encode::JP::2022_JP*
Encode::JP::JIS7 Internally used by Encode::JP
Encode::KR Korean Encodings
Encode::KR::2022_KR Internally used by Encode::KR
Encode::MIME::Header MIME ’B’ and ’Q’ header encoding
Encode::PerlIO A detailed document on Encode and PerlIO
Encode::Supported Encodings supported by Encode
Encode::Symbol Symbol Encodings
Encode::TW Taiwan-based Chinese Encodings
Encode::Unicode Various Unicode Transformation Formats
Encode::Unicode::UTF7 UTF-7 encoding
English Use nice English (or awk) names for ugly punctuation variables
Env Perl module that imports environment variables as scalars or arrays
Errno System errno constants
Exporter Implements default import method for modules
Exporter::Heavy Exporter guts
ExtUtils::Command Utilities to replace common UNIX commands in Makefiles etc.
ExtUtils::Command::MM Commands for the MM’s to use in Makefiles
ExtUtils::Constant Generate XS code to import C header constants
ExtUtils::Constant::Base Base class for ExtUtils::Constant objects
ExtUtils::Constant::Utils Helper functions for ExtUtils::Constant
ExtUtils::Constant::XS Base class for ExtUtils::Constant objects
ExtUtils::Embed Utilities for embedding Perl in C/C++ applications
ExtUtils::Install Install files from here to there
ExtUtils::Installed Inventory management of installed modules
ExtUtils::Liblist Determine libraries to use and how to use them
ExtUtils::MM OS adjusted ExtUtils::MakeMaker subclass
ExtUtils::MM_AIX AIX specific subclass of ExtUtils::MM_Unix
ExtUtils::MM_Any Platform-agnostic MM methods
ExtUtils::MM_BeOS Methods to override UN*X behaviour in ExtUtils::MakeMaker
ExtUtils::MM_Cygwin Methods to override UN*X behaviour in ExtUtils::MakeMaker
ExtUtils::MM_DOS DOS specific subclass of ExtUtils::MM_Unix
ExtUtils::MM_MacOS Once produced Makefiles for MacOS Classic
ExtUtils::MM_NW5 Methods to override UN*X behaviour in ExtUtils::MakeMaker
ExtUtils::MM_OS2 Methods to override UN*X behaviour in ExtUtils::MakeMaker
ExtUtils::MM_QNX QNX specific subclass of ExtUtils::MM_Unix
ExtUtils::MM_UWIN U/WIN specific subclass of ExtUtils::MM_Unix
ExtUtils::MM_Unix Methods used by ExtUtils::MakeMaker
ExtUtils::MM_VMS Methods to override UN*X behaviour in ExtUtils::MakeMaker
ExtUtils::MM_VOS VOS specific subclass of ExtUtils::MM_Unix
ExtUtils::MM_Win32 Methods to override UN*X behaviour in ExtUtils::MakeMaker
ExtUtils::MM_Win95 Method to customize MakeMaker for Win9X
ExtUtils::MY ExtUtils::MakeMaker subclass for customization
ExtUtils::MakeMaker Create a module Makefile
ExtUtils::MakeMaker::Config Wrapper around
ExtUtils::MakeMaker::FAQ Frequently Asked Questions About MakeMaker
ExtUtils::MakeMaker::Tutorial Writing a module with MakeMaker
ExtUtils::MakeMaker::bytes Version-agnostic
ExtUtils::MakeMaker::vmsish Platform-agnostic
ExtUtils::Manifest Utilities to write and check a MANIFEST file
ExtUtils::Mkbootstrap Make a bootstrap file for use by DynaLoader
ExtUtils::Mksymlists Write linker options files for dynamic extension
ExtUtils::Packlist Manage .packlist files
ExtUtils::testlib Add blib/* directories to @INC
Fatal Replace functions with equivalents which succeed or die
Fcntl Load the C Fcntl.h defines
File::Basename Parse file paths into directory, filename and suffix.
File::CheckTree Run many filetest checks on a tree
File::Compare Compare files or filehandles
File::Copy Copy files or filehandles
File::DosGlob DOS like globbing and then some
File::Find Traverse a directory tree.
File::Glob Perl extension for BSD glob routine
File::Path Create or remove directory trees
File::Spec Portably perform operations on file names
File::Spec::Cygwin Methods for Cygwin file specs
File::Spec::Epoc Methods for Epoc file specs
File::Spec::Functions Portably perform operations on file names
File::Spec::Mac File::Spec for Mac OS (Classic)
File::Spec::OS2 Methods for OS/2 file specs
File::Spec::Unix File::Spec for Unix, base for other File::Spec modules
File::Spec::VMS Methods for VMS file specs
File::Spec::Win32 Methods for Win32 file specs
File::Temp Return name and handle of a temporary file safely
File::stat By-name interface to Perl’s built-in stat() functions
FileCache Keep more files open than the system permits
FileHandle Supply object methods for filehandles
Filter::Simple Simplified source filtering
Filter::Util::Call Perl Source Filter Utility Module
FindBin Locate directory of original perl script
GDBM_File Perl5 access to the gdbm library.
Getopt::Long Extended processing of command line options
Getopt::Std Process single-character switches with switch clustering
Hash::Util A selection of general-utility hash subroutines
I18N::Collate Compare 8-bit scalar data according to the current locale
I18N::LangTags Functions for dealing with RFC3066-style language tags
I18N::LangTags::Detect Detect the user’s language preferences
I18N::LangTags::List Tags and names for human languages
I18N::Langinfo Query locale information
IO Load various IO modules
IO::Dir Supply object methods for directory handles
IO::File Supply object methods for filehandles
IO::Handle Supply object methods for I/O handles
IO::Pipe Supply object methods for pipes
IO::Poll Object interface to system poll call
IO::Seekable Supply seek based methods for I/O objects
IO::Select OO interface to the select system call
IO::Socket Object interface to socket communications
IO::Socket::INET Object interface for AF_INET domain sockets
IO::Socket::UNIX Object interface for AF_UNIX domain sockets
IPC::Open2 Open a process for both reading and writing
IPC::Open3 Open a process for reading, writing, and error handling
IPC::SysV SysV IPC constants
IPC::SysV::Msg SysV Msg IPC object class
IPC::SysV::Semaphore SysV Semaphore IPC object class
List::Util A selection of general-utility list subroutines
Locale::Constants Constants for Locale codes
Locale::Country ISO codes for country identification (ISO 3166)
Locale::Currency ISO three letter codes for currency identification (ISO 4217)
Locale::Language ISO two letter codes for language identification (ISO 639)
Locale::Maketext Framework for localization
Locale::Maketext::TPJ13 Article about software localization
Locale::Script ISO codes for script identification (ISO 15924)
MIME::Base64 Encoding and decoding of base64 strings
MIME::Base64::QuotedPrint Encoding and decoding of quoted-printable strings
Math::BigFloat Arbitrary size floating point math package
Math::BigInt Arbitrary size integer/float math package
Math::BigInt::Calc Pure Perl module to support Math::BigInt
Math::BigInt::CalcEmu Emulate low-level math with BigInt code
Math::BigRat Arbitrary big rational numbers
Math::Complex Complex numbers and associated mathematical functions
Math::Trig Trigonometric functions
Memoize Make functions faster by trading space for time
Memoize::AnyDBM_File Glue to provide EXISTS for AnyDBM_File for Storable use
Memoize::Expire Plug-in module for automatic expiration of memoized values
Memoize::ExpireFile Test for Memoize expiration semantics
Memoize::ExpireTest Test for Memoize expiration semantics
Memoize::NDBM_File Glue to provide EXISTS for NDBM_File for Storable use
Memoize::SDBM_File Glue to provide EXISTS for SDBM_File for Storable use
Memoize::Storable Store Memoized data in Storable database
NDBM_File Tied access to ndbm files
NEXT Provide a pseudo-class NEXT (et al) that allows method redispatch
Net::Cmd Network Command class (as used by FTP, SMTP etc)
Net::Config Local configuration data for libnet
Net::Domain Attempt to evaluate the current host’s internet name and domain
Net::FTP FTP Client class
Net::NNTP NNTP Client class
Net::Netrc OO interface to users netrc file
Net::POP3 Post Office Protocol 3 Client class (RFC1939)
Net::Ping Check a remote host for reachability
Net::SMTP Simple Mail Transfer Protocol Client
Net::Time Time and daytime network client interface
Net::hostent By-name interface to Perl’s built-in gethost*() functions
Net::libnetFAQ Libnet Frequently Asked Questions
Net::netent By-name interface to Perl’s built-in getnet*() functions
Net::protoent By-name interface to Perl’s built-in getproto*() functions
Net::servent By-name interface to Perl’s built-in getserv*() functions
O Generic interface to Perl Compiler backends
ODBM_File Tied access to odbm files
Opcode Disable named opcodes when compiling perl code
POSIX Perl interface to IEEE Std 1003.1
PerlIO On demand loader for PerlIO layers and root of PerlIO::* name space
PerlIO::encoding Encoding layer
PerlIO::scalar In-memory IO, scalar IO
PerlIO::via Helper class for PerlIO layers implemented in perl
PerlIO::via::QuotedPrint PerlIO layer for quoted-printable strings
Pod::Checker Check pod documents for syntax errors
Pod::Find Find POD documents in directory trees
Pod::Functions Group Perl’s functions a la perlfunc.pod
Pod::Html Module to convert pod files to HTML
Pod::InputObjects Objects representing POD input paragraphs, commands, etc.
Pod::LaTeX Convert Pod data to formatted Latex
Pod::Man Convert POD data to formatted *roff input
Pod::ParseLink Parse an L<> formatting code in POD text
Pod::ParseUtils Helpers for POD parsing and conversion
Pod::Parser Base class for creating POD filters and translators
Pod::Perldoc::ToChecker Let Perldoc check Pod for errors
Pod::Perldoc::ToMan Let Perldoc render Pod as man pages
Pod::Perldoc::ToNroff Let Perldoc convert Pod to nroff
Pod::Perldoc::ToPod Let Perldoc render Pod as ... Pod!
Pod::Perldoc::ToRtf Let Perldoc render Pod as RTF
Pod::Perldoc::ToText Let Perldoc render Pod as plaintext
Pod::Perldoc::ToTk Let Perldoc use Tk::Pod to render Pod
Pod::Perldoc::ToXml Let Perldoc render Pod as XML
Pod::PlainText Convert POD data to formatted ASCII text
Pod::Plainer Perl extension for converting Pod to old style Pod.
Pod::Select Extract selected sections of POD from input
Pod::Text Convert POD data to formatted ASCII text
Pod::Text::Color Convert POD data to formatted color ASCII text
Pod::Text::Overstrike Convert POD data to formatted overstrike text
Pod::Text::Termcap Convert POD data to ASCII text with format escapes
Pod::Usage Print a usage message from embedded pod documentation
SDBM_File Tied access to sdbm files
Safe Compile and execute code in restricted compartments
Scalar::Util A selection of general-utility scalar subroutines
Search::Dict Search for key in dictionary file
SelectSaver Save and restore selected file handle
SelfLoader Load functions only on demand
Shell Run shell commands transparently within perl
Socket Load the C socket.h defines and structure manipulators
Storable Persistence for Perl data structures
Switch A switch statement for Perl
Symbol Manipulate Perl symbols and their names
Sys::Hostname Try every conceivable way to get hostname
Sys::Syslog Perl interface to the UNIX syslog(3) calls
Term::ANSIColor Color screen output using ANSI escape sequences
Term::Cap Perl termcap interface
Term::Complete Perl word completion module
Term::ReadLine Perl interface to various readline packages.
Test Provides a simple framework for writing test scripts
Test::Builder Backend for building test libraries
Test::Builder::Module Base class for test modules
Test::Builder::Tester Test testsuites that have been built with
Test::Builder::Tester::Color Turn on colour in Test::Builder::Tester
Test::Harness Run Perl standard test scripts with statistics
Test::Harness::Assert Simple assert
Test::Harness::Iterator Internal Test::Harness Iterator
Test::Harness::Point Object for tracking a single test point
Test::Harness::Straps Detailed analysis of test results
Test::Harness::TAP Documentation for the TAP format
Test::More Yet another framework for writing test scripts
Test::Simple Basic utilities for writing tests.
Test::Tutorial A tutorial about writing really basic tests
Text::Abbrev Create an abbreviation table from a list
Text::Balanced Extract delimited text sequences from strings.
Text::ParseWords Parse text into an array of tokens or array of arrays
Text::Soundex Implementation of the Soundex Algorithm as Described by Knuth
Text::Tabs Expand and unexpand tabs per the unix expand(1) and unexpand(1)
Text::Wrap Line wrapping to form simple paragraphs
Thread Manipulate threads in Perl (for old code only)
Thread::Queue Thread-safe queues
Thread::Semaphore Thread-safe semaphores
Thread::Signal Start a thread which runs signal handlers reliably (for old code)
Thread::Specific Thread-specific keys
Tie::Array Base class for tied arrays
Tie::File Access the lines of a disk file via a Perl array
Tie::Handle Base class definitions for tied handles
Tie::Hash Base class definitions for tied hashes
Tie::Memoize Add data to hash when needed
Tie::RefHash Use references as hash keys
Tie::Scalar Base class definitions for tied scalars
Tie::SubstrHash Fixed-table-size, fixed-key-length hashing
Time::HiRes High resolution alarm, sleep, gettimeofday, interval timers
Time::Local Efficiently compute time from local and GMT time
Time::gmtime By-name interface to Perl’s built-in gmtime() function
Time::localtime By-name interface to Perl’s built-in localtime() function
Time::tm Internal object used by Time::gmtime and Time::localtime
UNIVERSAL Base class for ALL classes (blessed references)
Unicode::Collate Unicode Collation Algorithm
Unicode::Normalize Unicode Normalization Forms
Unicode::UCD Unicode character database
User::grent By-name interface to Perl’s built-in getgr*() functions
User::pwent By-name interface to Perl’s built-in getpw*() functions
XS::APItest Test the perl C API
XS::Typemap Module to test the XS typemaps distributed with perl
XSLoader Dynamically load C libraries into Perl code
To find out all modules installed on your system, including those without documentation or outside the standard release, just use the following command (under the default win32 shell, double quotes should be used instead of single quotes).

    % perl -MFile::Find=find -MFile::Spec::Functions -Tlwe \
      ’find { wanted => sub { print canonpath $_ if /\.pm\z/ },
      no_chdir => 1 }, @INC’

(The -T is here to prevent ’.’ from being listed in @INC.) They should all have their own documentation installed and accessible via your system man(1) command. If you do not have a find program, you can use the Perl find2perl program instead, which generates Perl code as output you can run through perl. If you have a man program but it doesn’t find your modules, you’ll have to fix your manpath. See perl for details. If you have no system man command, you might try the perldoc program.

Note also that the command perldoc perllocal gives you a (possibly incomplete) list of the modules that have been further installed on your system. (The perllocal.pod file is updated by the standard MakeMaker install process.)

Extension Modules

Extension modules are written in C (or a mix of Perl and C). They are usually dynamically loaded into Perl if and when you need them, but may also be linked in statically. Supported extension modules include Socket, Fcntl, and POSIX.

Many popular C extension modules do not come bundled (at least, not completely) due to their sizes, volatility, or simply lack of time for adequate testing and configuration across the multitude of platforms on which Perl was beta-tested. You are encouraged to look for them on CPAN (described below), or using web search engines like Alta Vista or Google.


CPAN stands for Comprehensive Perl Archive Network; it’s a globally replicated trove of Perl materials, including documentation, style guides, tricks and traps, alternate ports to non-Unix systems and occasional binary distributions for these. Search engines for CPAN can be found at

Most importantly, CPAN includes around a thousand unbundled modules, some of which require a C compiler to build. Major categories of modules are:
o Language Extensions and Documentation Tools
o Development Support
o Operating System Interfaces
o Networking, Device Control (modems) and InterProcess Communication
o Data Types and Data Type Utilities
o Database Interfaces
o User Interfaces
o Interfaces to / Emulations of Other Programming Languages
o File Names, File Systems and File Locking (see also File Handles)
o String Processing, Language Text Processing, Parsing, and Searching
o Option, Argument, Parameter, and Configuration File Processing
o Internationalization and Locale
o Authentication, Security, and Encryption
o World Wide Web, HTML, HTTP, CGI, MIME
o Server and Daemon Utilities
o Archiving and Compression
o Images, Pixmap and Bitmap Manipulation, Drawing, and Graphing
o Mail and Usenet News
o Control Flow Utilities (callbacks and exceptions etc)
o File Handle and Input/Output Stream Utilities
o Miscellaneous Modules
The list of the registered CPAN sites as of this writing follows. Please note that the sorting order is alphabetical on fields:


and thus the North American servers happen to be listed between the European and the South American sites.

You should try to choose one close to you.


South Africa













Russian Federation


Saudi Arabia




South Korea






Central America

Costa Rica







Bosnia and Herzegovina






Czech Republic




















































United Kingdom


North America





Nova Scotia






United States








District of Columbia














New Jersey


New York


North Carolina























New Zealand


United States


South America







RSYNC Mirrors


For an up-to-date listing of CPAN sites, see or .

Modules: Creation, Use, and Abuse

(The following section is borrowed directly from Tim Bunce’s modules file, available at your nearest CPAN site.)

Perl implements a class using a package, but the presence of a package doesn’t imply the presence of a class. A package is just a namespace. A class is a package that provides subroutines that can be used as methods. A method is just a subroutine that expects, as its first argument, either the name of a package (for static methods), or a reference to something (for virtual methods).

A module is a file that (by convention) provides a class of the same name (sans the .pm), plus an import method in that class that can be called to fetch exported symbols. This module may implement some of its methods by loading dynamic C or C++ objects, but that should be totally transparent to the user of the module. Likewise, the module might set up an AUTOLOAD function to slurp in subroutine definitions on demand, but this is also transparent. Only the .pm file is required to exist. See perlsub, perltoot, and AutoLoader for details about the AUTOLOAD mechanism.

Guidelines for Module Creation

o Do similar modules already exist in some form?

If so, please try to reuse the existing modules either in whole or by inheriting useful features into a new class. If this is not practical try to get together with the module authors to work on extending or enhancing the functionality of the existing modules. A perfect example is the plethora of packages in perl4 for dealing with command line options.

If you are writing a module to expand an already existing set of modules, please coordinate with the author of the package. It helps if you follow the same naming scheme and module interaction scheme as the original author.

o Try to design the new module to be easy to extend and reuse.

Try to use warnings; (or use warnings qw(...);). Remember that you can add no warnings qw(...); to individual blocks of code that need less warnings.

Use blessed references. Use the two argument form of bless to bless into the class name given as the first parameter of the constructor, e.g.,:

 sub new {
     my $class = shift;
     return bless {}, $class;

or even this if you’d like it to be used as either a static or a virtual method.

 sub new {
     my $self  = shift;
     my $class = ref($self) || $self;
     return bless {}, $class;

Pass arrays as references so more parameters can be added later (it’s also faster). Convert functions into methods where appropriate. Split large methods into smaller more flexible ones. Inherit methods from other modules if appropriate.

Avoid class name tests like: die "Invalid" unless ref $ref eq FOO . Generally you can delete the eq FOO part with no harm at all. Let the objects look after themselves! Generally, avoid hard-wired class names as far as possible.

Avoid $r->Class::func() where using @ISA=qw(... Class ...) and $r->func() would work (see perlbot for more details).

Use autosplit so little used or newly added functions won’t be a burden to programs that don’t use them. Add test functions to the module after __END__ either using AutoSplit or by saying:

 eval join(’’,<main::DATA>) || die $@ unless caller();

Does your module pass the ’empty subclass’ test? If you say @SUBCLASS::ISA = qw(YOURCLASS); your applications should be able to use SUBCLASS in exactly the same way as YOURCLASS. For example, does your application still work if you change: $obj = new YOURCLASS; into: $obj = new SUBCLASS; ?

Avoid keeping any state information in your packages. It makes it difficult for multiple other packages to use yours. Keep state information in objects.

Always use -w.

Try to use strict; (or use strict qw(...);). Remember that you can add no strict qw(...); to individual blocks of code that need less strictness.

Always use -w.

Follow the guidelines in the perlstyle(1) manual.

Always use -w.

o Some simple style guidelines

The perlstyle manual supplied with Perl has many helpful points.

Coding style is a matter of personal taste. Many people evolve their style over several years as they learn what helps them write and maintain good code. Here’s one set of assorted suggestions that seem to be widely used by experienced developers:

Use underscores to separate words. It is generally easier to read $var_names_like_this than $VarNamesLikeThis, especially for non-native speakers of English. It’s also a simple rule that works consistently with VAR_NAMES_LIKE_THIS.

Package/Module names are an exception to this rule. Perl informally reserves lowercase module names for ’pragma’ modules like integer and strict. Other modules normally begin with a capital letter and use mixed case with no underscores (need to be short and portable).

You may find it helpful to use letter case to indicate the scope or nature of a variable. For example:

 $ALL_CAPS_HERE   constants only (beware clashes with Perl vars)
 $Some_Caps_Here  package-wide global/static
 $no_caps_here    function scope my() or local() variables

Function and method names seem to work best as all lowercase. e.g., $obj->as_string().

You can use a leading underscore to indicate that a variable or function should not be used outside the package that defined it.

o Select what to export.

Do NOT export method names!

Do NOT export anything else by default without a good reason!

Exports pollute the namespace of the module user. If you must export try to use @EXPORT_OK in preference to @EXPORT and avoid short or common names to reduce the risk of name clashes.

Generally anything not exported is still accessible from outside the module using the ModuleName::item_name (or $blessed_ref->method) syntax. By convention you can use a leading underscore on names to indicate informally that they are ’internal’ and not for public use.

(It is actually possible to get private functions by saying: my $subref = sub { ... }; &$subref;. But there’s no way to call that directly as a method, because a method must have a name in the symbol table.)

As a general rule, if the module is trying to be object oriented then export nothing. If it’s just a collection of functions then @EXPORT_OK anything but use @EXPORT with caution.

o Select a name for the module.

This name should be as descriptive, accurate, and complete as possible. Avoid any risk of ambiguity. Always try to use two or more whole words. Generally the name should reflect what is special about what the module does rather than how it does it. Please use nested module names to group informally or categorize a module. There should be a very good reason for a module not to have a nested name. Module names should begin with a capital letter.

Having 57 modules all called Sort will not make life easy for anyone (though having 23 called Sort::Quick is only marginally better :-). Imagine someone trying to install your module alongside many others. If in any doubt ask for suggestions in comp.lang.perl.misc.

If you are developing a suite of related modules/classes it’s good practice to use nested classes with a common prefix as this will avoid namespace clashes. For example: Xyz::Control, Xyz::View, Xyz::Model etc. Use the modules in this list as a naming guide.

If adding a new module to a set, follow the original author’s standards for naming modules and the interface to methods in those modules.

If developing modules for private internal or project specific use, that will never be released to the public, then you should ensure that their names will not clash with any future public module. You can do this either by using the reserved Local::* category or by using a category name that includes an underscore like Foo_Corp::*.

To be portable each component of a module name should be limited to 11 characters. If it might be used on MS-DOS then try to ensure each is unique in the first 8 characters. Nested modules make this easier.

o Have you got it right?

How do you know that you’ve made the right decisions? Have you picked an interface design that will cause problems later? Have you picked the most appropriate name? Do you have any questions?

The best way to know for sure, and pick up many helpful suggestions, is to ask someone who knows. Comp.lang.perl.misc is read by just about all the people who develop modules and it’s the best place to ask.

All you need to do is post a short summary of the module, its purpose and interfaces. A few lines on each of the main methods is probably enough. (If you post the whole module it might be ignored by busy people - generally the very people you want to read it!)

Don’t worry about posting if you can’t say when the module will be ready - just say so in the message. It might be worth inviting others to help you, they may be able to complete it for you!

o README and other Additional Files.

It’s well known that software developers usually fully document the software they write. If, however, the world is in urgent need of your software and there is not enough time to write the full documentation please at least provide a README file containing:
o A description of the module/package/extension etc.
o A copyright notice - see below.
o Prerequisites - what else you may need to have.
o How to build it - possible changes to Makefile.PL etc.
o How to install it.
o Recent changes in this release, especially incompatibilities
o Changes / enhancements you plan to make in the future.

If the README file seems to be getting too large you may wish to split out some of the sections into separate files: INSTALL, Copying, ToDo etc.
o Adding a Copyright Notice.

How you choose to license your work is a personal decision. The general mechanism is to assert your Copyright and then make a declaration of how others may copy/use/modify your work.

Perl, for example, is supplied with two types of licence: The GNU GPL and The Artistic Licence (see the files README, Copying, and Artistic, or perlgpl and perlartistic). Larry has good reasons for NOT just using the GNU GPL.

My personal recommendation, out of respect for Larry, Perl, and the Perl community at large is to state something simply like:

 Copyright (c) 1995 Your Name. All rights reserved.
 This program is free software; you can redistribute it and/or
 modify it under the same terms as Perl itself.

This statement should at least appear in the README file. You may also wish to include it in a Copying file and your source files. Remember to include the other words in addition to the Copyright.

o Give the module a version/issue/release number.

To be fully compatible with the Exporter and MakeMaker modules you should store your module’s version number in a non-my package variable called $VERSION. This should be a floating point number with at least two digits after the decimal (i.e., hundredths, e.g, $VERSION = "0.01"). Don’t use a 1.3.2 style version. See Exporter for details.

It may be handy to add a function or method to retrieve the number. Use the number in announcements and archive file names when releasing the module (ModuleName-1.02.tar.Z). See perldoc for details.

o How to release and distribute a module.

It’s good idea to post an announcement of the availability of your module (or the module itself if small) to the comp.lang.perl.announce Usenet newsgroup. This will at least ensure very wide once-off distribution.

If possible, register the module with CPAN. You should include details of its location in your announcement.

Some notes about ftp archives: Please use a long descriptive file name that includes the version number. Most incoming directories will not be readable/listable, i.e., you won’t be able to see your file after uploading it. Remember to send your email notification message as soon as possible after uploading else your file may get deleted automatically. Allow time for the file to be processed and/or check the file has been processed before announcing its location.

FTP Archives for Perl Modules:

Follow the instructions and links on:

or upload to one of these sites:

and notify <>.

By using the WWW interface you can ask the Upload Server to mirror your modules from your ftp or WWW site into your own directory on CPAN!

Please remember to send me an updated entry for the Module list!

o Take care when changing a released module.

Always strive to remain compatible with previous released versions. Otherwise try to add a mechanism to revert to the old behavior if people rely on it. Document incompatible changes.

Guidelines for Converting Perl 4 Library Scripts into Modules

o There is no requirement to convert anything.

If it ain’t broke, don’t fix it! Perl 4 library scripts should continue to work with no problems. You may need to make some minor changes (like escaping non-array @’s in double quoted strings) but there is no need to convert a .pl file into a Module for just that.

o Consider the implications.

All Perl applications that make use of the script will need to be changed (slightly) if the script is converted into a module. Is it worth it unless you plan to make other changes at the same time?

o Make the most of the opportunity.

If you are going to convert the script to a module you can use the opportunity to redesign the interface. The guidelines for module creation above include many of the issues you should consider.

o The pl2pm utility will get you started.

This utility will read *.pl files (given as parameters) and write corresponding *.pm files. The pl2pm utilities does the following:
o Adds the standard Module prologue lines
o Converts package specifiers from ’ to ::
o Converts die(...) to croak(...)
o Several other minor changes

Being a mechanical process pl2pm is not bullet proof. The converted code will need careful checking, especially any package statements. Don’t delete the original .pl file till the new .pm one works!

Guidelines for Reusing Application Code

o Complete applications rarely belong in the Perl Module Library.
o Many applications contain some Perl code that could be reused.

Help save the world! Share your code in a form that makes it easy to reuse.

o Break-out the reusable code into one or more separate module files.
o Take the opportunity to reconsider and redesign the interfaces.
o In some cases the ’application’ can then be reduced to a small

fragment of code built on top of the reusable modules. In these cases the application could invoked as:

     % perl -e ’use Module::Name; method(@ARGV)’ ...
     % perl -mModule::Name ...    (in perl5.002 or higher)


Perl does not enforce private and public parts of its modules as you may have been used to in other languages like C++, Ada, or Modula-17. Perl doesn’t have an infatuation with enforced privacy. It would prefer that you stayed out of its living room because you weren’t invited, not because it has a shotgun.

The module and its user have a contract, part of which is common law, and part of which is written. Part of the common law contract is that a module doesn’t pollute any namespace it wasn’t asked to. The written contract for the module (A.K.A. documentation) may make other provisions. But then you know when you use RedefineTheWorld that you’re redefining the world and willing to take the consequences.

perl v5.8.8 PERLMODLIB (1) 2011-06-13
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