Home > Articles > Web Development > Perl

Modules in Perl

  • Print
  • + Share This
Make your Perl programs more maintainable and reusable. Learn how to effectively use and document packages and modules.
This chapter is from the book

This chapter is from the book

Over the years, Perl has grown to include many new features. These features make the language richer and more useful—but they also make it more complex for programmers and the Perl maintainers.

Perl 5 partly solved this problem by making the language extensible with modules that range widely in complexity, adding anything from convenience variables to sophisticated database clients and Web development environments. Modules have made it possible for Perl to improve incrementally without changing the language itself.

This chapter begins with a discussion of Perl packages, which allow us to place variables and subroutines in a namespace hierarchy. Once we have discussed packages, we begin to discuss modules—how to use them, how to write them, and how to improve them.

By the end of this chapter, you should understand not just how Perl modules work, but also how you can use them effectively in your programs.

6.1 Packages

Programmers working on large projects often discover that a variable or subroutine name is being used by someone else. Perl and other languages provide packages, or namespaces, which make it easier to avoid such clashes. Packages are analogous to surnames in human society, allowing more than one David or Jennifer to coexist unambiguously.

6.1.1 Packages

Every global variable in Perl exists within a package, with the default package being main. The global variable $x is actually shorthand for $main::x, where main is the package, $x is the variable, and :: separates the package name from the unqualified variable name.

We can similarly refer to variables in other packages. For example, $fruit::mango, @fruit::kiwi, and %fruit::apple are all in the fruit package. As you can see, symbols representing a data type ($, @, or %) precede the package name, not the unqualified variable name. As with variables, packages spring to life when they are first referenced.

Package names may contain ::, allowing us to create what appear to be hierarchies. For instance, $fruit::tropical::kiwi is the variable $kiwi in the package fruit::tropical. However, these names are only significant to programmers; Perl does not notice or enforce hierarchies. As far as Perl is concerned, two unrelated modules can be under the same package hierarchy, and two related modules can be in completely different packages.

At any time in our program, we can set or retrieve the value of any global variable by giving its fully qualified name:

$main::x = 5; 
$blueberry::x = 30; 

print "main::x = $main::x\n"; 
print "blueberry::x = $blueberry::x\n"; 

6.1.2 Lexicals and packages

Lexicals exist outside of a package, in a separate area known as the scratchpad. They have nothing to do with packages or global variables. There is no relationship between $main::var and the lexical $var, except in the mind of a programmer. This program is perfectly legal, but hard for programmers to understand:

#!/usr/bin/perl	 	 
# filename: globals-and-lexicals.pl	 
 	 	 	 
use warnings;	 	 
 	 	 	 
$	main::x = 10;	              # Global	 
my $x = 20;	                      # Lexical	 
 	 	 	 
print "x = '$x'\n";	              # Prints 20 (lexical)
print "main::x = '$main::x'\n";	  # Prints 10 (global)

Once the lexical $x is declared, $main::x must be retrieved with its fully qualified name. Otherwise, Perl will assume that $x refers to the lexical $x, rather than the global $main::x.

6.1.3 use strict

use strict tells the Perl compiler to forbid the use of unqualified global variables, avoiding the ambiguity that we saw in the preceding program. When use strict is active, $x must refer to a lexical explicitly declared with my. If no such lexical has been declared, the program exits with a compilation error:

#!/usr/bin/perl 
# filename: counter.pl 


use strict; 
use warnings; 


# Declare $counter lexical within the 
foreach loop foreach my $counter (0 .. 10) 
{ 
   print "Counter = $counter\n"; 
   $counter++; 
} 

# $counter has disappeared -- fatal compilation error! 
print "Counter at the end is $counter\n"; 

We can fix this program by declaring $counter to be a top-level lexical:

#!/usr/bin/perl 
# filename: new-counter.pl 


use strict; 

use warnings; 


# Declare $counter to be lexical for the entire program 
my $counter; 


# Declare $index to be lexical within the foreach 
foreach my $index (0 .. 10) 
{ 
   print "Counter = $counter\n"; 
   $counter++; 
} 

# Counter still exists 
print "Counter at the end is $counter\n"; 

6.1.4 use vars and our

Experienced Perl programmers include use strict in their programs, because of the number of errors it traps. However, referring to globals by their full names quickly gets tedious.

use vars helps by making an exception to use strict. Variables named in the list passed to use vars can be referred to by their unqualified names, even when use strict is active. For example, the following code tells Perl that $a, $b, and $c in the current package do not need to be fully qualified:

use vars qw($a $b $c); 

In the case of a conflict between my and use vars, the lexical has priority. (After all, you can always set and retrieve the global's value using its fully qualified name, but the lexical has only one name.) The following program demonstrates this:

#!/usr/bin/perl 
# filename: globals-and-lexicals-2.pl 

use strict; 
use warnings; 
use vars qw($x);     # Allows us to write $main::x as $x 


$x = 10;             # Sets global $main::x 
my $x = 20;          # Sets lexical $x 
$x = 30;             # Sets lexical $x, not global $main::x 


print "x = '$x'\n";              # Prints 30 (lexical) 
print "main::x = '$main::x'\n";  # Prints 10 (global) 

As of Perl 5.6, use vars has been deprecated in favor of our. our is similar to my, in that its declarations only last through the current lexical scope. However, our (like use vars) works with global variables, not lexicals. We can rewrite this program as follows using our:

#!/usr/bin/perl 
# filename: globals-and-lexicals-with-our.pl 


use strict; 
use warnings; 
our $x;               # Allows us to write $main::x as $x

$x = 10;              # Sets global $main::
my $x = 20; x         # Sets lexical $x
$x = 30;              # Sets lexical $x, not global $main::x


print "x = '$x'\n";              # Prints 30 (lexical) 
print "main::x = '$main::x'\n";  # Prints 10 (global) 

6.1.5 Switching default packages

To change to a new default package, use the package statement:

package newPackageName; 

A program can change default packages as often as it might like, although doing so can confuse the next person maintaining your code. Remember that package changes the default namespace; it does not change your ability to set or receive any global's value by explicitly naming its package.

There is a subtle difference between use vars and our that comes into play when we change packages. use vars ceases to have effect when you change to a different default package. For example:

package foo;       # Make the default package 'foo' 
use vars qw($x);   # $x is shorthand for $foo::x 

$x = 5;            # Assigns $foo::x 

package bar;       # Make the default package 'bar' 
print "'$x'\n";    # $x refers to $bar::x (undefined) 

package foo;       # Make the default package 'foo' (again) 
print "'$x'\n";    # $x refers to $foo::x 

In this code, use vars tells Perl that $x is shorthand for $foo::x. When we switch into default package bar, $x no longer refers to $foo::x, but $bar::x. Without use strict, Perl allows us to retrieve the value of an undeclared global variable, which has the value undef. When we return to package foo, $x once again refers to $foo::x, and the previous value is once again available.

By contrast, global variables declared with our remain available with their short names even after changing into a different package:

package foo;       # Make the default package 'foo' 
our $x;            # $x is shorthand for $foo::x 

$x = 5;            # Assigns $foo::x 

package bar;       # Make the default package 'bar' 
print "'$x'\n";    # $x still refers to $foo::x 

For $x to refer to $bar::x, we must add an additional our declaration immediately following the second package statement:

package foo;       # Make the default package 'foo' 
our $x;            # $x is shorthand for $foo::x 

$x = 5;            # Assigns $foo::x 

package bar;       # Make the default package 'bar' 

our $x;	           # $x is now shorthand for $bar::x	 
print "'$x'\n";	   # $x now refers to $bar::x	 

6.1.6 Subroutines

When we declare a subroutine, it is placed by default in the current package:

package abc;	 	 
sub foo {return 5;}        # Full name is abc::foo	 	 

This means that when working with more than one package, we may need to qualify subroutine names:

package numbers;           # Default package is "numbers"
sub gimme_five { 5; }      # Define a subroutine	 
 	 	 
package main;              # Default package is now "main"
 	 	 
my $x = gimme_five();	   # Fails to execute main::gimme_five()
print "x = '$x'\n";	       # Prints 5	 

This code exits with a fatal error, with Perl complaining that no subroutine main::gimme five has been defined. We can fix this by invoking the subroutine with its fully qualified name:

Package numbers;                 # Default package is "numbers"
sub gimme_five { 5; }            # Define	a subroutine
 	
package main;                    # Default package is now "main"
 
my $ x = numbers::gimme_five();  # Qualify gimme_five() with a package
print "x	= '$x'\n";           # Prints	5
  • + 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.

Overview


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.

Surveys

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.

Newsletters

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.

Security


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

Children


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

Marketing


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.

Choice/Opt-out


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.

Links


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