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PERLMODLIB(1) Perl Programmers Reference Guide 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).

Get/set subroutine or variable attributes
Replace functions with ones that succeed or die with lexical scope
Exceptions from autodying functions.
Exceptions from autodying system().
Provide hints about user subroutines to autodie
Skip a package when throwing autodie exceptions
Postpone load of modules until a function is used
Establish an ISA relationship with base classes at compile time
Transparent BigInteger support for Perl
Transparent BigNumber support for Perl
Transparent BigNumber/BigRational support for Perl
Use MakeMaker's uninstalled version of a package
Expose the individual bytes of characters
Access to Unicode character names and named character sequences; also define character names
Declare constants
Perl pragma for deprecating the inclusion of a module in core
Produce verbose warning diagnostics
Allows you to write your script in non-ASCII and non-UTF-8
Warn on implicit encoding conversions
Experimental features made easy
Enable new features
Compile-time class fields
Control the filetest permission operators
"use" a Perl module if a condition holds
Use integer arithmetic instead of floating point
Request less of something
Manipulate @INC at compile time
Use or avoid POSIX locales for built-in operations
Method Resolution Order
Alternative to Test::More::use_ok
Set default PerlIO layers for input and output
Restrict unsafe operations when compiling
Package for overloading Perl operations
Lexically control overloading
Establish an ISA relationship with base classes at compile time
Alter regular expression behaviour
Enable simple signal handling
Control sort() behaviour
Restrict unsafe constructs
Predeclare subroutine names
Perl interpreter-based threads
Perl extension for sharing data structures between threads
Enable/disable UTF-8 (or UTF-EBCDIC) in source code
Predeclare global variable names
Perl extension for Version Objects
Control VMS-specific language features
Control optional warnings
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.

Perl extension for ARexx support
Perl extension for low level amiga support
Provide framework for multiple DBMs
Easily interact with CPAN from the command line
Implements the "prove" command.
State storage for the "prove" command.
Individual test suite results.
Individual test results.
Module for manipulations of tar archives
A subclass for in-memory extracted file from Archive::Tar
Simpler definition of attribute handlers
Load subroutines only on demand
Split a package for autoloading
The Perl Compiler Backend
Walk Perl syntax tree, printing concise info about ops
Perl compiler backend to produce perl code
OP op_private flag definitions
Show lexical variables used in functions or files
Walk Perl syntax tree, printing terse info about ops
Generates cross reference reports for Perl programs
Benchmark running times of Perl code
Family-neutral IP socket supporting both IPv4 and IPv6
Networking constants and support functions
Namespace for Perl's core routines
Query, download and build perl modules from CPAN sites
A recipe book for programming with
Internal debugging for
Read and match distroprefs
Utility for CPAN::Config file Initialization
Internal configuration handling for
Interface between and
The distribution metadata for a CPAN dist
Convert CPAN distribution metadata structures
An optional feature provided by a CPAN distribution
History of CPAN Meta Spec changes
Version 1.0 metadata specification for META.yml
Version 1.1 metadata specification for META.yml
Version 1.2 metadata specification for META.yml
Version 1.3 metadata specification for META.yml
Version 1.4 metadata specification for META.yml
Merging CPAN Meta fragments
A set of distribution prerequisites by phase and type
A set of version requirements for a CPAN dist
Specification for CPAN distribution metadata
Validate CPAN distribution metadata structures
Read and write a subset of YAML for CPAN Meta files
Wrapper around without using any XS module
Base class for CPAN shell extensions
Proof of concept implementation of a trivial CPAN::Plugin
Internal queue support for
Internal handling of tar archives for
Utility functions to compare CPAN versions
Alternative warn and die for modules
Declare struct-like datatypes as Perl classes
Low-Level Interface to bzip2 compression library
Low-Level Interface to zlib compression library
Interface to zlib compression library
Access Perl configuration information
Hash lookup of which core extensions were built.
Structured data retrieval of perl -V output
Get pathname of current working directory
Programmatic interface to the Perl debugging API
Filter DBM keys/values
Filter for DBM_Filter
Filter for DBM_Filter
Filter for DBM_Filter
Filter for DBM_Filter
Filter for DBM_Filter
Perl5 access to Berkeley DB version 1.x
Stringified perl data structures, suitable for both printing and "eval"
A data debugging tool for the XS programmer
Generate stubs for a SelfLoading module
Modules that calculate message digests
Perl interface to the MD5 Algorithm
Perl extension for SHA-1/224/256/384/512
Digest base class
Calculate digests of files
(obsolete) supply object methods for directory handles
Provides screen dump of Perl data.
Dynamically load C libraries into Perl code
Character encodings in Perl
Alias definitions to encodings
Single Byte Encodings
Internally used by Encode::??::ISO_2022_*
China-based Chinese Encodings
Internally used by Encode::CN
Internally used by Encode
EBCDIC Encodings
Object Oriented Encoder
Encode Implementation Base Class
ESTI GSM 03.38 Encoding
Guesses encoding from data
Japanese Encodings
Internally used by Encode::JP::2022_JP*
Internally used by Encode::JP
Korean Encodings
Internally used by Encode::KR
MIME encoding for an unstructured email header
Internally used by Encode
A detailed document on Encode and PerlIO
Encodings supported by Encode
Symbol Encodings
Taiwan-based Chinese Encodings
Various Unicode Transformation Formats
UTF-7 encoding
Use nice English (or awk) names for ugly punctuation variables
Perl module that imports environment variables as scalars or arrays
System errno constants
Implements default import method for modules
Exporter guts
Compile and link C code for Perl modules
Builder class for Windows platforms
Utilities to replace common UNIX commands in Makefiles etc.
Commands for the MM's to use in Makefiles
Generate XS code to import C header constants
Base class for ExtUtils::Constant objects
Helper functions for ExtUtils::Constant
Generate C code for XS modules' constants.
Utilities for embedding Perl in C/C++ applications
Install files from here to there
Inventory management of installed modules
Determine libraries to use and how to use them
OS adjusted ExtUtils::MakeMaker subclass
AIX specific subclass of ExtUtils::MM_Unix
Platform-agnostic MM methods
Methods to override UN*X behaviour in ExtUtils::MakeMaker
Methods to override UN*X behaviour in ExtUtils::MakeMaker
DOS specific subclass of ExtUtils::MM_Unix
Special behaviors for OS X
Once produced Makefiles for MacOS Classic
Methods to override UN*X behaviour in ExtUtils::MakeMaker
Methods to override UN*X behaviour in ExtUtils::MakeMaker
QNX specific subclass of ExtUtils::MM_Unix
U/WIN specific subclass of ExtUtils::MM_Unix
Methods used by ExtUtils::MakeMaker
Methods to override UN*X behaviour in ExtUtils::MakeMaker
VOS specific subclass of ExtUtils::MM_Unix
Methods to override UN*X behaviour in ExtUtils::MakeMaker
Method to customize MakeMaker for Win9X
ExtUtils::MakeMaker subclass for customization
Create a module Makefile
Wrapper around
Frequently Asked Questions About MakeMaker
Bundled Encode::Locale
Writing a module with MakeMaker
Utilities to write and check a MANIFEST file
Write the C code for miniperlmain.c and perlmain.c
Make a bootstrap file for use by DynaLoader
Write linker options files for dynamic extension
Manage .packlist files
Converts Perl XS code into C code
Initialization values for some globals
Clean package to evaluate code in
Subroutines used with ExtUtils::ParseXS
Read/Write/Modify Perl/XS typemap files
Quick commands for handling typemaps
Entry in the INPUT section of a typemap
Entry in the OUTPUT section of a typemap
Entry in the TYPEMAP section of a typemap
Keep sets of symbol names palatable to the VMS linker
Add blib/* directories to @INC
Replace functions with equivalents which succeed or die
Load the C Fcntl.h defines
Parse file paths into directory, filename and suffix.
Compare files or filehandles
Copy files or filehandles
DOS like globbing and then some
A generic file fetching mechanism
Traverse a directory tree.
Perl extension for BSD glob routine
Extend File Glob to Allow Input and Output Files
Create or remove directory trees
Portably perform operations on file names
File::Spec for AmigaOS
Methods for Cygwin file specs
Methods for Epoc file specs
Portably perform operations on file names
File::Spec for Mac OS (Classic)
Methods for OS/2 file specs
File::Spec for Unix, base for other File::Spec modules
Methods for VMS file specs
Methods for Win32 file specs
Return name and handle of a temporary file safely
By-name interface to Perl's built-in stat() functions
Keep more files open than the system permits
Supply object methods for filehandles
Simplified source filtering
Perl Source Filter Utility Module
Locate directory of original perl script
Perl5 access to the gdbm library.
Extended processing of command line options
Process single-character switches with switch clustering
A small, simple, correct HTTP/1.1 client
A selection of general-utility hash subroutines
Support for Inside-Out Classes
Compare 8-bit scalar data according to the current locale
Functions for dealing with RFC3066-style language tags
Detect the user's language preferences
Tags and names for human languages
Query locale information
Load various IO modules
Base Class for IO::Compress modules
Write bzip2 files/buffers
Write RFC 1950 files/buffers
Frequently Asked Questions about IO::Compress
Write RFC 1952 files/buffers
Write RFC 1951 files/buffers
Write zip files/buffers
Supply object methods for directory handles
Supply object methods for filehandles
Supply object methods for I/O handles
Supply object methods for pipes
Object interface to system poll call
Supply seek based methods for I/O objects
OO interface to the select system call
Object interface to socket communications
Object interface for AF_INET domain sockets
Object interface for AF_UNIX domain sockets
Uncompress zlib-based (zip, gzip) file/buffer
Uncompress gzip, zip, bzip2, xz, lzma, lzip, lzf or lzop file/buffer
Base Class for IO::Uncompress modules
Read bzip2 files/buffers
Read RFC 1952 files/buffers
Read RFC 1950 files/buffers
Read RFC 1951 files/buffers
Read zip files/buffers
IO:: style interface to Compress::Zlib
Finding and running system commands made easy
SysV Msg IPC object class
Open a process for both reading and writing using open2()
Open a process for reading, writing, and error handling using open3()
SysV Semaphore IPC object class
SysV Shared Memory IPC object class
System V IPC constants and system calls
Reserved special namespace for internals related functions
JSON::XS compatible pure-Perl module.
Dummy module providing JSON::PP::Boolean
A selection of general-utility list subroutines
Indicate if List::Util was compiled with a C compiler
Framework for localization
Recipes for using Locale::Maketext
Deprecated module to load Locale::Maketext utf8 code
Deprecated module to load Locale::Maketext utf8 code
Simple interface to Locale::Maketext::Lexicon
Article about software localization
Encoding and decoding of base64 strings
Encoding and decoding of quoted-printable strings
Arbitrary size floating point math package
Arbitrary size integer/float math package
Pure Perl module to support Math::BigInt
Math::BigInt::Calc with some XS for more speed
Virtual parent class for Math::BigInt libraries
Arbitrary big rational numbers
Complex numbers and associated mathematical functions
Trigonometric functions
Make functions faster by trading space for time
Glue to provide EXISTS for AnyDBM_File for Storable use
Plug-in module for automatic expiration of memoized values
Test for Memoize expiration semantics
Test for Memoize expiration semantics
Glue to provide EXISTS for NDBM_File for Storable use
Glue to provide EXISTS for SDBM_File for Storable use
Store Memoized data in Storable database
What modules shipped with versions of perl
What utilities shipped with versions of perl
Runtime require of both modules and files
Looking up module information / loading at runtime
Mark modules as loaded or unloaded
Gather package and POD information from perl module files
Tied access to ndbm files
Provide a pseudo-class NEXT (et al) that allows method redispatch
Network Command class (as used by FTP, SMTP etc)
Local configuration data for libnet
Attempt to evaluate the current host's internet name and domain
FTP Client class
FTP Client data connection class
NNTP Client class
OO interface to users netrc file
Post Office Protocol 3 Client class (RFC1939)
Check a remote host for reachability
Simple Mail Transfer Protocol Client
Time and daytime network client interface
By-name interface to Perl's built-in gethost*() functions
Libnet Frequently Asked Questions
By-name interface to Perl's built-in getnet*() functions
By-name interface to Perl's built-in getproto*() functions
By-name interface to Perl's built-in getserv*() functions
Generic interface to Perl Compiler backends
Tied access to odbm files
Disable named opcodes when compiling perl code
Perl interface to IEEE Std 1003.1
A generic input parsing/checking mechanism.
Parse META.yml and META.json CPAN metadata files
Map Perl operating system names to generic types
On demand loader for PerlIO layers and root of PerlIO::* name space
Encoding layer
Memory mapped IO
In-memory IO, scalar IO
Helper class for PerlIO layers implemented in perl
PerlIO layer for quoted-printable strings
Check pod documents for syntax errors
For resolving Pod E<...> sequences
Group Perl's functions a la perlfunc.pod
Module to convert pod files to HTML
Convert POD data to formatted *roff input
Parse an L<> formatting code in POD text
Look up Perl documentation in Pod format.
Base for Pod::Perldoc formatters
Customized option parser for Pod::Perldoc
Render Pod with ANSI color escapes
Let Perldoc check Pod for errors
Let Perldoc render Pod as man pages
Let Perldoc convert Pod to nroff
Let Perldoc render Pod as ... Pod!
Let Perldoc render Pod as RTF
Render Pod with terminal escapes
Let Perldoc render Pod as plaintext
Let Perldoc use Tk::Pod to render Pod
Let Perldoc render Pod as XML
Framework for parsing Pod
Check the Pod syntax of a document
Put Pod::Simple into trace/debug mode
Dump Pod-parsing events as text
Turn Pod into XML
Convert Pod to HTML
Convert several Pod files to several HTML files
Just the Pod, the whole Pod, and nothing but the Pod
Represent "section" attributes of L codes
Turn Pod::Simple events into method calls
A pull-parser interface to parsing Pod
End-tokens from Pod::Simple::PullParser
Start-tokens from Pod::Simple::PullParser
Text-tokens from Pod::Simple::PullParser
Tokens from Pod::Simple::PullParser
Format Pod as RTF
Find POD documents in directory trees
Parse Pod into a simple parse tree
Write a formatter as a Pod::Simple subclass
Format Pod as plaintext
Get the text content of Pod
Format Pod as validating XHTML
Turn Pod into XML
Convert POD data to formatted text
Convert POD data to formatted color ASCII text
Convert POD data to formatted overstrike text
Convert POD data to ASCII text with format escapes
Print a usage message from embedded pod documentation
Tied access to sdbm files
Compile and execute code in restricted compartments
A selection of general-utility scalar subroutines
Look - search for key in dictionary file
Save and restore selected file handle
Load functions only on demand
Persistence for Perl data structures
A selection of utility subroutines for subs and CODE references
Manipulate Perl symbols and their names
Try every conceivable way to get hostname
Perl interface to the UNIX syslog(3) calls
Win32 support for Sys::Syslog
Base class that provides common functionality to TAP::Parser
Base class for harness output delegates
Run Perl test scripts with color
Harness output delegate for default console output
Harness output delegate for parallel console output
Harness output delegate for default console output
Harness output delegate for file output
Harness output delegate for file output
Abstract base class for harness output delegate
Run test scripts with statistics
Parsing harness related environmental variables where appropriate
Base class that provides common functionality to all "TAP::*" modules
Parse TAP output
Aggregate TAP::Parser results
A grammar for the Test Anything Protocol.
Base class for TAP source iterators
Iterator for array-based TAP sources
Iterator for process-based TAP sources
Iterator for filehandle-based TAP sources
Figures out which SourceHandler objects to use for a given Source
Multiplex multiple TAP::Parsers
Base class for TAP::Parser output objects
Bailout result token.
Comment result token.
Plan result token.
TAP pragma token.
Test result token.
Unknown result token.
TAP syntax version token.
YAML result token.
Factory for creating TAP::Parser output objects
Schedule tests during parallel testing
A single testing job.
A no-op job.
A TAP source & meta data about it
Base class for different TAP source handlers
Stream output from an executable TAP source
Stream TAP from a text file.
Stream TAP from an IO::Handle or a GLOB.
Stream TAP from a Perl executable
Stream output from raw TAP in a scalar/array ref.
Read YAMLish data from iterator
Write YAMLish data
Color screen output using ANSI escape sequences
Perl termcap interface
Perl word completion module
Perl interface to various "readline" packages.
Provides a simple framework for writing test scripts
Framework for writing test tools that all work together.
Primary interface for writing Test2 based testing tools.
What breaks at what version
Object to represent a testing context.
Object used by Test2::API under the hood
Object to manage a stack of Test2::Hub
Base class for events
Diag event type
Set the encoding for the output stream
Exception event
Event for a simple failed assertion
Generic event type.
Note event type
Ok event type
Event for a simple passing assertion
The event of a plan
Skip event type
Event for subtest types
Event for TAP version.
Second generation event.
Tell all procs/threads it is time to be done
Base class for all event facets.
Facet with event details.
Facet for assertion amnesty.
Facet representing an assertion.
Facet for hub actions and behaviors.
Facet for errors that need to be shown.
Facet for the hubs an event passes through.
Facet for information a developer might care about.
Intermediary representation of a table.
Facet for meta-data
Facet for events contains other events
Facet for setting the plan
Facet that dictates how to render an event.
Debug information for events
Namespace for formatters.
Standard TAP formatter
The conduit through which all events flow.
Hub used by interceptor to grab results.
Exception class used by
Hub used by subtests
Turn on IPC for threading or forking support.
Base class for Test2 IPC drivers.
Temp dir + Files concurrency model.
Tiny set of tools for unfortunate souls who cannot use
Transition notes when upgrading to Test2
Tools used by Test2 and friends.
Allow third party tools to safely attach meta-data
Convert facet data to the legacy event API.
Build hash based classes.
Legacy wrapper fro Test2::EventFacet::Trace.
Backend for building test libraries
Test::Builder subclass of Test2::Formatter::TAP
A copy of IO::Scalar for Test::Builder
Base class for test modules
Test testsuites that have been built with
Turn on colour in Test::Builder::Tester
Test::Builder subclass of Test2::Event::Diag
Run Perl standard test scripts with statistics
Beyond make test
Yet another framework for writing test scripts
Basic utilities for writing tests.
Ease testing test modules built with Test::Builder
Help testing test modules built with Test::Builder
Help testing test modules built with Test::Builder
A tutorial about writing really basic tests
Alternative to Test::More::use_ok
Abbrev - create an abbreviation table from a list
Extract delimited text sequences from strings.
Parse text into an array of tokens or array of arrays
Expand and unexpand tabs like unix expand(1) and unexpand(1)
Line wrapping to form simple paragraphs
Manipulate threads in Perl (for old code only)
Thread-safe queues
Thread-safe semaphores
Base class for tied arrays
Access the lines of a disk file via a Perl array
Base class definitions for tied handles
Base class definitions for tied hashes
Named regexp capture buffers
Add data to hash when needed
Use references as hash keys
Base class definitions for tied scalars
Base class definitions for tied handles
Fixed-table-size, fixed-key-length hashing
High resolution alarm, sleep, gettimeofday, interval timers
Efficiently compute time from local and GMT time
Object Oriented time objects
A simple API to convert seconds to other date values
By-name interface to Perl's built-in gmtime() function
By-name interface to Perl's built-in localtime() function
Internal object used by Time::gmtime and Time::localtime
Base class for ALL classes (blessed references)
Unicode Collation Algorithm
Weighting CJK Unified Ideographs
Weighting CJK Unified Ideographs
Weighting JIS KANJI for Unicode::Collate
Weighting CJK Unified Ideographs
Weighting CJK Unified Ideographs
Weighting CJK Unified Ideographs
Weighting CJK Unified Ideographs
Linguistic tailoring for DUCET via Unicode::Collate
Unicode Normalization Forms
Unicode character database
By-name interface to Perl's built-in getgr*() functions
By-name interface to Perl's built-in getpw*() functions
Perl extension to manipulate DCL symbols
Convert between VMS and Unix file specification syntax
Standard I/O functions via VMS extensions
Interfaces to some Win32 API Functions
Low-level access to Win32 system API calls for files/dirs.
Win32 CORE function stubs
Test the perl C API
Module to test the XS typemaps distributed with perl
Dynamically load C libraries into Perl code
Wrapper class for calling subs at end of scope
Hook stack for managing scopes via %^H
Internal Utility subroutines for autodie and Fatal
Perl extension for Version Objects

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 Google or DuckDuckGo.


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:

  • Language Extensions and Documentation Tools
  • Development Support
  • Operating System Interfaces
  • Networking, Device Control (modems) and InterProcess Communication
  • Data Types and Data Type Utilities
  • Database Interfaces
  • User Interfaces
  • Interfaces to / Emulations of Other Programming Languages
  • File Names, File Systems and File Locking (see also File Handles)
  • String Processing, Language Text Processing, Parsing, and Searching
  • Option, Argument, Parameter, and Configuration File Processing
  • Internationalization and Locale
  • Authentication, Security, and Encryption
  • World Wide Web, HTML, HTTP, CGI, MIME
  • Server and Daemon Utilities
  • Archiving and Compression
  • Images, Pixmap and Bitmap Manipulation, Drawing, and Graphing
  • Mail and Usenet News
  • Control Flow Utilities (callbacks and exceptions etc)
  • File Handle and Input/Output Stream Utilities
  • Miscellaneous Modules

The list of the registered CPAN sites 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.

Registered CPAN sites




North America


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, perlobj, and AutoLoader for details about the AUTOLOAD mechanism.

Guidelines for Module Creation

  • 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.

  • 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.

    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: "$obj = YOURCLASS->new();" into: "$obj = SUBCLASS->new();" ?

    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 perlstyle.

    Always use -w.

  • 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.

  • 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.

  • 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 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.

    For additional guidance on the naming of modules, please consult:

    or send mail to the <> mailing list.

  • 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. The <> mailing list is useful for this purpose; it's also accessible via news interface as perl.module-authors at

    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!

  • 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:

  • A description of the module/package/extension etc.
  • A copyright notice - see below.
  • Prerequisites - what else you may need to have.
  • How to build it - possible changes to Makefile.PL etc.
  • How to install it.
  • Recent changes in this release, especially incompatibilities
  • 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.

  • 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.

  • 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 $VERSION. This should be a positive floating point number with at least two digits after the decimal (i.e., hundredths, e.g, "$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.

  • How to release and distribute a module.

    If possible, register the module with CPAN. Follow the instructions and links on:

    and upload to:

    and notify <>. This will allow anyone to install your module using the "cpan" tool distributed with Perl.

    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!

  • 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

  • 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.

  • 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?

  • 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.

  • 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:

  • Adds the standard Module prologue lines
  • Converts package specifiers from ' to ::
  • Converts die(...) to croak(...)
  • 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

  • Complete applications rarely belong in the Perl Module Library.
  • 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.

  • Break-out the reusable code into one or more separate module files.
  • Take the opportunity to reconsider and redesign the interfaces.
  • 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.

2021-09-24 perl v5.32.1