perlmod - Perl modules (packages)



Perl provides a mechanism for alternative namespaces to protect packages from stomping on each other's variables. In fact, apart from certain magical variables, there's really no such thing as a global variable in Perl. The package statement declares the compilation unit as being in the given namespace. The scope of the package declaration is from the declaration itself through the end of the enclosing block (the same scope as the local() operator). All further unqualified dynamic identifiers will be in this namespace. A package statement only affects dynamic variables--including those you've used local() on--but not lexical variables created with my(). Typically it would be the first declaration in a file to be included by the require or use operator. You can switch into a package in more than one place; it merely influences which symbol table is used by the compiler for the rest of that block. You can refer to variables and filehandles in other packages by prefixing the identifier with the package name and a double colon: $Package::Variable. If the package name is null, the main package is assumed. That is, $::sail is equivalent to $main::sail.

(The old package delimiter was a single quote, but double colon is now the preferred delimiter, in part because it's more readable to humans, and in part because it's more readable to emacs macros. It also makes C++ programmers feel like they know what's going on.)

Packages may be nested inside other packages: $OUTER::INNER::var. This implies nothing about the order of name lookups, however. All symbols are either local to the current package, or must be fully qualified from the outer package name down. For instance, there is nowhere within package OUTER that $INNER::var refers to $OUTER::INNER::var. It would treat package INNER as a totally separate global package.

Only identifiers starting with letters (or underscore) are stored in a package's symbol table. All other symbols are kept in package main, including all of the punctuation variables like $_. In addition, the identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, INC and SIG are forced to be in package main, even when used for other purposes than their built-in one. Note also that, if you have a package called m, s or y, then you can't use the qualified form of an identifier because it will be interpreted instead as a pattern match, a substitution, or a translation.

(Variables beginning with underscore used to be forced into package main, but we decided it was more useful for package writers to be able to use leading underscore to indicate private variables and method names. $_ is still global though.)

Eval()ed strings are compiled in the package in which the eval() was compiled. (Assignments to $SIG{}, however, assume the signal handler specified is in the main package. Qualify the signal handler name if you wish to have a signal handler in a package.) For an example, examine in the Perl library. It initially switches to the DB package so that the debugger doesn't interfere with variables in the script you are trying to debug. At various points, however, it temporarily switches back to the main package to evaluate various expressions in the context of the main package (or wherever you came from). See the perldebug manpage .

See the perlsub manpage for other scoping issues related to my() and local(), or the perlref manpage regarding closures.

Symbol Tables

The symbol table for a package happens to be stored in the associative array of that name appended with two colons. The main symbol table's name is thus %main::, or %:: for short. Likewise symbol table for the nested package mentioned earlier is named %OUTER::INNER::.

The value in each entry of the associative array is what you are referring to when you use the *name typeglob notation. In fact, the following have the same effect, though the first is more efficient because it does the symbol table lookups at compile time:

local(*main::foo) = *main::bar; local($main::{'foo'}) = $main::{'bar'};

You can use this to print out all the variables in a package, for instance. Here is from the Perl library:

package dumpvar; sub main::dumpvar { ($package) = @_; local(*stab) = eval("*${package}::"); while (($key,$val) = each(%stab)) { local(*entry) = $val; if (defined $entry) { print "\$$key = '$entry'\n"; } if (defined @entry) { print "\@$key = (\n"; foreach $num ($[ .. $#entry) { print " $num\t'",$entry[$num],"'\n"; } print ")\n"; } if ($key ne "${package}::" && defined %entry) { print "\%$key = (\n"; foreach $key (sort keys(%entry)) { print " $key\t'",$entry{$key},"'\n"; } print ")\n"; } } }

Note that even though the subroutine is compiled in package dumpvar, the name of the subroutine is qualified so that its name is inserted into package main.

Assignment to a typeglob performs an aliasing operation, i.e.,

*dick = *richard;

causes variables, subroutines and file handles accessible via the identifier richard to also be accessible via the identifier dick. If you only want to alias a particular variable or subroutine, you can assign a reference instead:

*dick = \$richard;

makes $richard and $dick the same variable, but leaves @richard and @dick as separate arrays. Tricky, eh?

This mechanism may be used to pass and return cheap references into or from subroutines if you won't want to copy the whole thing.

%some_hash = (); *some_hash = fn( \%another_hash ); sub fn { local *hashsym = shift; # now use %hashsym normally, and you # will affect the caller's %another_hash my %nhash = (); # do what you want return \%nhash; }

On return, the reference wil overwrite the hash slot in the symbol table specified by the *some_hash typeglob. This is a somewhat tricky way of passing around references cheaply when you won't want to have to remember to dereference variables explicitly.

Another use of symbol tables is for making ``constant'' scalars.

*PI = \3.14159265358979;

Now you cannot alter $PI, which is probably a good thing all in all.

Package Constructors and Destructors

There are two special subroutine definitions that function as package constructors and destructors. These are the BEGIN and END routines. The sub is optional for these routines.

A BEGIN subroutine is executed as soon as possible, that is, the moment it is completely defined, even before the rest of the containing file is parsed. You may have multiple BEGIN blocks within a file--they will execute in order of definition. Because a BEGIN block executes immediately, it can pull in definitions of subroutines and such from other files in time to be visible to the rest of the file.

An END subroutine is executed as late as possible, that is, when the interpreter is being exited, even if it is exiting as a result of a die() function. (But not if it's is being blown out of the water by a signal--you have to trap that yourself (if you can).) You may have multiple END blocks within a file--they will execute in reverse order of definition; that is: last in, first out (LIFO).

Inside an END subroutine $? contains the value that the script is going to pass to exit(). You can modify $? to change the exit value of the script. Beware of changing $? by accident (eg, by running something via system).

Note that when you use the -n and -p switches to Perl, BEGIN and END work just as they do in awk, as a degenerate case.

Perl Classes

There is no special class syntax in Perl, but a package may function as a class if it provides subroutines that function as methods. Such a package may also derive some of its methods from another class package by listing the other package name in its @ISA array.

For more on this, see the perlobj manpage .

Perl Modules

A module is just a package that is defined in a library file of the same name, and is designed to be reusable. It may do this by providing a mechanism for exporting some of its symbols into the symbol table of any package using it. Or it may function as a class definition and make its semantics available implicitly through method calls on the class and its objects, without explicit exportation of any symbols. Or it can do a little of both.

For example, to start a normal module called Fred, create a file called and put this at the start of it:

package Fred; use Exporter (); @ISA = qw(Exporter); @EXPORT = qw(func1 func2); @EXPORT_OK = qw($sally @listabob %harry func3);

Then go on to declare and use your variables in functions without any qualifications. See Exporter and the Perl Modules File for details on mechanics and style issues in module creation.

Perl modules are included into your program by saying

use Module;


use Module LIST;

This is exactly equivalent to

BEGIN { require ""; import Module; }


BEGIN { require ""; import Module LIST; }

As a special case

use Module ();

is exactly equivalent to

BEGIN { require ""; }

All Perl module files have the extension .pm. use assumes this so that you don't have to spell out ``'' in quotes. This also helps to differentiate new modules from old .pl and .ph files. Module names are also capitalized unless they're functioning as pragmas, ``Pragmas'' are in effect compiler directives, and are sometimes called ``pragmatic modules'' (or even ``pragmata'' if you're a classicist).

Because the use statement implies a BEGIN block, the importation of semantics happens at the moment the use statement is compiled, before the rest of the file is compiled. This is how it is able to function as a pragma mechanism, and also how modules are able to declare subroutines that are then visible as list operators for the rest of the current file. This will not work if you use require instead of use. With require you can get into this problem:

require Cwd; # make Cwd:: accessible $here = Cwd::getcwd(); use Cwd; # import names from Cwd:: $here = getcwd(); require Cwd; # make Cwd:: accessible $here = getcwd(); # oops! no main::getcwd()

In general use Module (); is recommended over require Module;.

Perl packages may be nested inside other package names, so we can have package names containing ::. But if we used that package name directly as a filename it would makes for unwieldy or impossible filenames on some systems. Therefore, if a module's name is, say, Text::Soundex, then its definition is actually found in the library file Text/

Perl modules always have a .pm file, but there may also be dynamically linked executables or autoloaded subroutine definitions associated with the module. If so, these will be entirely transparent to the user of the module. It is the responsibility of the .pm file to load (or arrange to autoload) any additional functionality. The POSIX module happens to do both dynamic loading and autoloading, but the user can just say use POSIX to get it all.

For more information on writing extension modules, see the perlxs manpage and the perlguts manpage .


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 (AKA 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.


A number of modules are included the the Perl distribution. These are described below, and all end in .pm. 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 module.) The pl2pm file in the distribution may help in your conversion, but it's just a mechanical process, so is far from bulletproof.

Pragmatic Modules

They work somewhat like pragmas in that they tend to affect the compilation of your program, and thus will usually only work well when used within a use, or no. These are locally scoped, so an inner BLOCK may countermand any of these by saying

no integer; no strict 'refs';

which lasts until the end of that BLOCK.

The following programs are defined (and have their own documentation).

Pragma to produce enhanced diagnostics

Pragma to compute arithmetic in integer instead of double

Pragma to request less of something from the compiler

Pragma to restrict use of unsafe opcodes

Pragma for overloading operators

Pragma to enable stack backtrace on unexpected signals

Pragma to restrict unsafe constructs

Pragma to predeclare sub names

Pragma to predeclare global symbols

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.

provide framework for multiple DBMs

load functions only on demand

split a package for autoloading

benchmark running times of code

warn of errors (from perspective of caller)

access Perl configuration option

get pathname of current working directory

Perl access to Berkeley DB

generate stubs for a SelfLoading module

Dynamically load C libraries into Perl code

use nice English (or awk) names for ugly punctuation variables

perl module that imports environment variables

provide import/export controls for Perl modules

determine libraries to use and how to use them

create an extension Makefile

utilities to write and check a MANIFEST file

make a bootstrap file for use by DynaLoader


load the C Fcntl.h defines

parse file specifications

run many filetest checks on a tree

traverse a file tree

supply object methods for filehandles

create or remove a series of directories

extended getopt processing

Process single-character switches with switch clustering

compare 8-bit scalar data according to the current locale

a process for both reading and writing

open a process for reading, writing, and error handling

check a host for upness

Perl interface to IEEE Std 1003.1

load functions only on demand

Creation controlled compartments in which perl code can be evaluated.

load the C socket.h defines and structure manipulators

run perl standard test scripts with statistics

create an abbreviation table from a list

To find out all the modules installed on your system, includingthose without documentation or outside the standard release, do this:

find `perl -e 'print "@INC"'` -name '*.pm' -print

They should all have their own documentation installed and accessible via your system man(1) command. If that fails, try the perldoc program.

Extension Modules

Extension modules are written in C (or a mix of Perl and C) and get dynamically loaded into Perl if and when you need them. Supported extension modules include the Socket, Fcntl, and POSIX modules.

Many popular C extension modules do not come bundled (at least, not completely) due to their size, 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 in archie(1L), the Perl FAQ or Meta-FAQ, the WWW page, and even with their authors before randomly posting asking for their present condition and disposition.


CPAN stands for the Comprehensive Perl Archive Network. This is a globally replicated collection of all known Perl materials, including hundreds of unbundled modules. Here are the major categories of modules:

The registered CPAN sites as of this writing include the following.You should try to choose one close to you:

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

Modules: Creation, Use and Abuse

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

Perl 5 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.

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.
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 hardwired class names as far as possible.

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

Use autosplit so little used or newly added functions won't be a burden to programs which 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 sub-class' 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 which need less strictness. Always use -w. Always use -w! Follow the guidelines in the perlstyle(1) manual.

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 informally indicate 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, since 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 informally group or categorise a module. A module should have a very good reason 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.

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

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!

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:

If the README file seems to be getting too large you may wish tosplit 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 license: The GNU GPL and The Artistic License (see the files README, Copying and Artistic). 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 simply state something 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 valid floating point number with at least two digits after the decimal (ie hundredths, e.g, $VERSION = ``0.01''). Don't use a ``1.3.2'' style version. See in Perl5.001m or later 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.
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 you should place the module into a major ftp archive and include details of it's location in your announcement.

Some notes about ftp archives: Please use a long descriptive file name which 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!

Take care when changing a released module.
Always strive to remain compatible with previous released versions (see 2.2 above) Otherwise try to add a mechanism to revert to the old behaviour 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 the perl applications which 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:

Being a mechanical process pl2pm is not bullet proof. The convertedcode 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 which 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)' ... or perl -mModule::Name ... (in perl5.002)