Packages in Perl

A Perl package is a collection of code which resides in its own namespace. Perl module is a package defined in a file having the same name as that of the package and having extension .pm. Two different modules may contain a variable or a function of the same name. Any variable which is not contained in any package belongs to the main package. Therefore, all the variables being used, belong to the ‘main’ package. With the declaration of additional packages, it is maintained that variables in different packages do not interfere with each other.

Declaration of a Perl Module
Name of the module must be the same as that of the package name and has a .pm extension.

Example : Calculator.pm

package Calculator;

# Defining sub-routine for Addition
sub addition
{
    # Initializing Variables a & b
    $a = $_[0];
    $b = $_[1];

    # Performing the operation
    $a = $a + $b;

    # Function to print the Sum
    print "\n***Addition is $a";
}

# Defining sub-routine for Subtraction
sub subtraction
{
    # Initializing Variables a & b
    $a = $_[0];
    $b = $_[1];

    # Performing the operation
    $a = $a - $b;

    # Function to print the difference
    print "\n***Subtraction is $a";
}
1;

Here, the name of the file is “Calculator.pm” stored in the directory Calculator. Notice that 1; is written at the end of the code to return a true value to the interpreter. Perl accepts anything which is true instead of 1.

Using a Perl Module
To use this calculator module, we use require or use functions. To access a function or a variable from a module, :: is used. Here is an example demonstrating the same:

Examples: Test.pl

#!/usr/bin/perl

# Using the Package 'Calculator'
use Calculator;

print "Enter two numbers to add";

# Defining values to the variables
$a = 10;
$b = 20;

# Subroutine call
Calculator::addition($a, $b);

print "\nEnter two numbers to subtract";

# Defining values to the variables
$a = 30;
$b = 10;

# Subroutine call
Calculator::subtraction($a, $b);

Output:

Accessing a Package from a different directory
If a file accessing the package lies outside the directory then we use ‘::’ to tell the path of the module. For example, file using calculator module is outside the calculator package so we write Calculator :: Calculator to load the module, where the value on left of ‘::’ represents package name and value on the right of ‘::’ represents Perl module name. Let’s see an example to understand this:

Examples: Test_out_package.pl outside Calculator directory

#!/usr/bin/perl

use GFG::Calculator; # Directory_name::module_name

print "Enter two numbers to add";

# Defining values to the variables
$a = 10;
$b = 20;

# Subroutine call
Calculator::addition($a, $b);

print "\nEnter two numbers to subtract";

# Defining values to the variables
$a = 30;
$b = 10;

# Subroutine call
Calculator::subtraction($a, $b);

Output:

Using Variables from modules
Variables from different packages can be used by declaring them before using. Following example demonstrates this
Examples: Message.pm

#!/usr/bin/perl

package Message;

# Variable Creation
$username;

# Defining subroutine
sub Hello
{
  print "Hello $username\n";
}
1;

Perl file to access the module is as below
Examples: variable.pl

#!/usr/bin/perl

# Using Message.pm package
use Message;

# Defining value to variable
$Message::username = "ABC";

# Subroutine call
Message::Hello();

Output:

Begin and End Block
BEGIN and END block is used when we want to run some part of the code at the beginning and some part of the code at an end. The codes written within BEGIN{…} are executed at the beginning of the script while the codes written within END{…} are executed at the end of the script. The program below demonstrates this:
Examples: begineg.pl

#!/usr/bin/perl

# Predefined BEGIN block
BEGIN
{
    print "In the begin block\n";
}

# Predefined END block
END
{
    print "In the end block\n";
}

print "Hello Perl;\n";

Output:

In the begin block
Hello Perl;
In the end block