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perlmod - Perl modules (packages)



Perl provides a mechanism for alternative namespaces to protect packages from stomping on each others variables. In fact, apart from certain magical variables, there's really no such thing as a global variable in Perl. By default, a Perl script starts compiling into the package known as main. You can switch namespaces using the package declaration. The scope of the package declaration is from the declaration itself to the end of the enclosing block (the same scope as the local() operator). Typically it would be the first declaration in a file to be included by the require 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 as 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. 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.)

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 perldb.pl 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 .

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 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 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 dumpvar.pl 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 symbol table entry 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 symbol 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?

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).

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 5, 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

In Perl 5, the notion of packages has been extended into the notion of modules. A module is 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.

Perl modules are included by saying

use Module;


use Module LIST;

This is exactly equivalent to

BEGIN { require "Module.pm"; import Module; }


BEGIN { require "Module.pm"; import Module LIST; }

All Perl module files have the extension .pm. use assumes this so that you don't have to spell out "Module.pm" 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 . Therefore, if you're planning on the module altering your namespace, use use ; otherwise, use require . Otherwise 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()

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/Soundex.pm.

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 perlapi 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 bullet proof.

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).

Perl pragma to compute arithmetic in integer instead of double

Perl pragma to request less of something from the compiler

Perl pragma to enable stack backtrace on unexpected signals

Perl pragma to restrict unsafe constructs

Perl pragma to predeclare sub names


Standard Modules

The following 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.

create an abbreviation table from a list

provide framework for multiple DBMs

load functions only on demand

split a package for autoloading

parse file name and path from a specification

benchmark running times of code

warn or die of errors (from perspective of caller)

run many filetest checks on a tree

compare 8-bit scalar data according to the current locale

access Perl configuration option

get pathname of current working directory

Dynamically load C libraries into Perl code

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

Perl module that imports environment variables

module to control namespace manipulations

load the C Fcntl.h defines

supply object methods for filehandles

traverse a file tree

traverse a directory structure depth-first

basic and extended getopt(3) processing

generate a Makefile for Perl extension

open a process for both reading and writing

open a process for reading, writing, and error handling

Perl interface to IEEE 1003.1 namespace

check a host for upness

load the C socket.h defines


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.

The following are popular C extension modules, which while available at Perl 5.0 release time, 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. There's no guarantee that the names or addresses below have not changed since printing, and in fact, they probably have!

Written by William Setzer<William_Setzer@ncsu.edu>, while not included with the standard distribution, this extension module ports to most systems. FTP from your nearest Perl archive site, or try


It is currently in alpha test, so the name and ftp location may change.

This is the portable database interface written by <Tim.Bunce@ig.co.uk>. This supersedes the many perl4 ports for database extensions. The official archive for DBperl extensions is ftp.demon.co.uk:/pub/perl/db. This archive contains copies of perl4 ports for Ingres, Oracle, Sybase, Informix, Unify, Postgres, and Interbase, as well as rdb and shql and other non-SQL systems.

Fastest and most restriction-free of the DBM bindings, this extension module uses the popular Berkeley DB to tie() into your hashes. This has a standardly-distributed man page and dynamic loading extension module, but you'll have to fetch the Berkeley code yourself. See DB_File for where.

This extension module is a front to the Athena and Xlib libraries for Perl GUI programming, originally written by by Dominic Giampaolo <dbg@sgi.com>, then and rewritten for Sx by Frédéric Chauveau<fmc@pasteur.fr>. It's available for FTP from


This extension module is an object-oriented Perl5 binding to the popular tcl/tk X11 package. However, you need know no TCL to use it! It was written by Malcolm Beattie<mbeattie@sable.ox.ac.uk>. If you are unable to locate it using archie(1L) or a similar tool, you may try retrieving it from /private/Tk-october.tar.gz from Malcolm's machine listed above.