Home > Articles > Web Development > Perl

Modules in Perl

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book

6.2 Modules

Modules make it easy to reuse Perl code in multiple programs. Writing a Perl module involves defining a set of variables and subroutines within a package, and then making those definitions available for public consumption. Just as you can avoid repeating code within a program by using subroutines, you can avoid repeating code within a set of programs by writing a module. In this section, we will see just how easy it is to create and use Perl modules.

6.2.1 Creating a module

To write a module, create a file with a .pm suffex. The file should define global variables and subroutines, all within a package with a name that is the same as the filename. If the file is named Foo.pm, then it should define variables and subroutines in the package Foo. (User-defined module and package names traditionally begin with capital letters.) If the package name contains ::, such as Food::Cooking, then the file should be named Cooking.pm, placed within a directory named Food. Perl translates the hierarchy separator :: into a directory separator, regardless of what platform you're running. Food::Cooking would thus be in Food/ Cooking.pm on a Unix system, and Food\Cooking.pm on a Windows system. Within a Perl program, these platform-specific differences are normally invisible.

A module will normally begin with a package statement, naming the package in which the module's variables and subroutines will be placed. So in Food::Cooking, the first line would be:

package Food::Cooking; 

The last line of a module must evaluate to true. This is traditionally done by putting the number 1 on the final line:


The following is a perfectly legal definition of the Mathtest module, which defines two variables and a subroutine:

use strict; 

package Mathtest; 
our ($pi, $e); 

$pi = 3.14159;   # Define $Mathtest::pi 
$e = 2.7182818;  # Define $Mathtest::e 

sub circle_area  # Declare a subroutine 
   # Get the argument to our subroutine 
   my $radius = shift; 
   # Return the calculated area 
   return ($pi * $radius * $radius); 
1;               # Loaded successfully

6.2.2 Using our module

Once Mathtest has been created, a program can use it with the compile-time use statement:

use Mathtest; 

A program can also use the -M command-line argument:

perl -MMathtest -e 'print "$Mathtest::pi\n";' 

This could be rewritten as:

perl -e 'use Mathtest; print "$Mathtest::pi\n";' 

6.2.3 Details of module loading

The array @INC lists the directories in which Perl should look for modules. When it encounters a use statement, Perl looks through the directories in @INC sequentially, stopping when it finds a match.

The following one-liner displays the contents of @INC:

perl -e 'print join "\n", @INC, "\n";' 

On my computer, this program displayed the following:


If Mathtest.pm is in any of these directories, then use Mathtest within our program will work. If Perl cannot find Mathtest.pm in any of these directories, the program exits with a fatal compilation error. . represents the current directory, meaning that Perl will normally look for modules in the same directory as an executing program. Activating taint mode (see Section 13.3) removes . from @INC, as a safety precaution.

A program can add a directory to @INC with use lib:

use lib "/usr/random/directory"; 

use lib adds directories to the front of @INC, giving them priority over the default directories. Command-line programs can have the same effect with -I:

perl -I/usr/local/perl/modules -I/home/joeuser/modules -MJoe -w 

When Perl finds and loads a module with use, it adds a key–value pair to %INC. Each key in %INC is a module name, and its corresponding value is the file from which it was loaded. This ensures that each module is loaded only once, even if it appears in two use statements.

6.2.4 Using modules

Once a module is loaded, its variables and subroutines are available to a program. For example:

use Mathtest;
my $log_base = $Mathtest::e; 
print "Log base: $log_base\n";      # Prints 2.7182818
my $radius = 10;
my $area =
print "Area = $area\n";             # Prints 314.159

Globals declared in a module are no different from any other global variables. We can thus modify global variables defined in a module, as the following code demonstrates:

use Mathtest;                       # Import the module
$Mathtest::pi = 4;                  # Redefine pi! 

my $radius = 10; 
my $area = Mathtest::circle_area($radius); 
print "$area\n";                    # Prints 400 

It is generally a bad idea to redefine variables defined by a module unless the module's documentation encourages such behavior explicitly.

6.2.5 Lexicals in modules

Lexical variables, as we have already seen, look similar to global variables but act quite differently. Every global is associated with a package and lasts until the program exits. By contrast, lexicals do not have any associated package and disappear when their enclosing block goes out of scope.

Because they lack a package, lexical variables defined at a module's top-level scope are invisible to programs outside of the module. This can sometimes be useful, but it usually leads to confusion—particularly for the programmer who cannot figure out why he or she cannot access a module's variables!

Understanding the difference between globals and lexicals is a key part of programming in Perl, and this is especially true when working with modules. Although it is normally a good idea to use lexicals wherever possible, this does not include module variables that must be visible to the outside world.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020