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Perl — 5

Himashi Karunathilake
11 min readAug 18, 2023

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Image from https://www.vectorlogo.zone/logos/perl/perl-ar21.svg
Link to Part 1: https://himashikarunathilake.medium.com/perl-1-5a5f4ec8c251
Link to Part 2: https://himashikarunathilake.medium.com/perl-2-12f31be96028
Link to Part 3: https://himashikarunathilake.medium.com/perl-3-daf8722151fa
Link to Part 4: https://himashikarunathilake.medium.com/perl-4-2215fa18efc3

Given below is the main.pl file that will be used to run all the sub files in this section:

#!/usr/bin/perl

# The fifth program in Perl.

use strict;
use warnings;
use feature "say";

say "";

say "*************** RUNNING THE MODULES.PL FILE ***************";
system("perl modules.pl");
say "__________________________________________________________________________________________";
say "";

say "*************** RUNNING THE ANONYMOUS_SUBROUTINES.PL FILE ***************";
system("perl anonymous_subroutines.pl");
say "__________________________________________________________________________________________";
say "";

say "*************** RUNNING THE FILE_HANDLING.PL FILE ***************";
system("perl file_handling.pl");
say "__________________________________________________________________________________________";
say "";

say "*************** RUNNING THE ERROR_HANDLING.PL FILE ***************";
system("perl error_handling.pl");
say "__________________________________________________________________________________________";
say "";

Modules in Perl

Both modules and packages in Perl are mechanisms used to organize code into separate files and namespaces, thereby helping to create modular, reusable, and maintainable code. A module is a collection of related functions, variables, and classes whereas a package is a namespace that encapsulates a set of variables and functions.

To create a module, a file with a “.pm” extension should be created with the same name as the module.

Given below is the Greet.pm module:

# Create the module Greet
package Greet;

# Define a subroutine - say_hello
sub say_hello {
    my ($name) = @_;
    print "Hello, $name!\n";
}

1;

Given below is the modules.pl script that will be using the module Greet created above:

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

# Add the current directory to @INC
use lib '.';

# Import the Greet module
use Greet;

say "=============== Welcome to the Greet Module! ===============";

print "Please provide your name: ";
my $name = <STDIN>;
chomp $name;

say "";

# Call a function from the module
Greet::say_hello($name);
Using a module in Perl

Anonymous Subroutines

In Perl, an anonymous subroutine, also known as a “lambda” or “closure” is a subroutine without a name and are useful for creating ad-hoc code blocks. It can be stored in a scalar variable or passed as an argument to another subroutine.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

# Create an anonymous subroutine and store it in a variable
my $add = sub {
    my ($num1, $num2) = @_;
    return $num1 + $num2;
};

say "=============== Performing Additions ===============";

print "Please provide the first number for the addition: ";
my $num1 = <STDIN>;
chomp $num1;
print "Please provide the second number for the addition: ";
my $num2 = <STDIN>;
chomp $num2;

# Call the anonymous subroutine
say "Calling the anonymous subroutine..........";
my $result = $add->($num1, $num2);
say "The addition of the two numbers is: $result";
Using an anonymous subroutine

File Handling in Perl

Reading from a File

Given below is the Perl code that can be used to read information from a file called input_file.txt:

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

say "=============== Reading from a File ===============";

print "Please provide the name of the input file: ";
my $inputFile = <STDIN>;
chomp $inputFile;

# Open the file in read mode
open(my $fh, '<', $inputFile) or die "ERROR! Could not open file: $!";

# Read the file line by line
while (my $line = <$fh>) {
    chomp $line;
    say $line;
}

# Close the file
close($fh);
Original input_file.txt
Reading the file input_file.txt

Appending to a File

Given below is the Perl code that can be used to append information to the file called input_file.txt:

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

say "=============== Reading from a File ===============";

print "Please provide the name of the input file: ";
my $inputFile = <STDIN>;
chomp $inputFile;

# Open the file in read mode
open(my $fh, '<', $inputFile) or die "ERROR! Could not open file: $!";

# Read the file line by line
while (my $line = <$fh>) {
    chomp $line;
    say $line;
}

# Close the file
close($fh);

say "";
say "";
say "";

say "=============== Appending to a File ===============";

print "Please provide the name of the input file: ";
my $appendFile = <STDIN>;
chomp $appendFile;

# Open the file in append mode
open($fh, '>>', $appendFile) or die "ERROR! Could not open file: $!";

# Append to the file
print "Please provide the text to append to the file: ";
my $text = <STDIN>;
chomp $text;
say $fh $text;

# Close the file
close($fh);

say "";
say "SUCCESS! The text has been appended to the file. Please find the modified file contents below:";
say "";
say "";

# Open the file again in read mode
open($fh, '<', $appendFile) or die "ERROR! Could not open file: $!";

# Read and print the modified file contents
while (my $line = <$fh>) {
    chomp $line;
    say $line;
}

# Close the file
close($fh);
Appending to input_file.txt
input_file.txt with appended text

Writing to a File

Given below is the Perl code that can be used to write information to a file called output_file.txt:

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

say "=============== Reading from a File ===============";

print "Please provide the name of the input file: ";
my $inputFile = <STDIN>;
chomp $inputFile;

# Open the file in read mode
open(my $fh, '<', $inputFile) or die "ERROR! Could not open file: $!";

# Read the file line by line
while (my $line = <$fh>) {
    chomp $line;
    say $line;
}

# Close the file
close($fh);

say "";
say "";
say "";

say "=============== Appending to a File ===============";

print "Please provide the name of the input file: ";
my $appendFile = <STDIN>;
chomp $appendFile;

# Open the file in append mode
open($fh, '>>', $appendFile) or die "ERROR! Could not open file: $!";

# Append to the file
print "Please provide the text to append to the file: ";
my $text = <STDIN>;
chomp $text;
say $fh $text;

# Close the file
close($fh);

say "";
say "SUCCESS! The text has been appended to the file. Please find the modified file contents below:";
say "";
say "";

# Open the file again in read mode
open($fh, '<', $appendFile) or die "ERROR! Could not open file: $!";

# Read and print the modified file contents
while (my $line = <$fh>) {
    chomp $line;
    say $line;
}

# Close the file
close($fh);

say "";
say "";
say "";
say "=============== Writing to a File ===============";

print "Please provide the name of the output file: ";
my $outputFile = <STDIN>;
chomp $outputFile;

# Open the file in write mode
open($fh, '>', $outputFile) or die "ERROR! Could not open file: $!";

# Read the input from the user
print 'Please provide the text to write to the file (provide your input as a single line by using \n for line breaks): ';
$text = <STDIN>;
chomp $text;

# Replace "\n" with actual newline characters
$text =~ s/\\n/\n/g;

# Write to the file
say $fh $text;

# Close the file
close($fh);

say "";
say "SUCCESS! The text has been written to the file.";
Writing to output.txt
output_file.txt

Error Handling

die Function

In Perl, the “die” function is used to terminate the program and display an error message when a critical error occurs.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

# die Function
say "To open an existing file, please provide the input file name as error_handling.txt.\nTo try out the \"die\" function, please provide the input file name as non_existent_file.txt.";
print "\nPlease provide the name of the input file: ";
my $dieInputFile= <STDIN>;
chomp $dieInputFile;

say "";

# Open the file in read mode
open(my $fh, '<', $dieInputFile) or die "ERROR! Could not open file: $!";
say "Please find the file contents below: ";
say "";

# Read the file line by line
while (my $line = <$fh>) {
    chomp $line;
    say $line;
}
Output when the input is a file that exists (error_handling.txt)
Output when the input is a file that does not exist (non_existent_file.txt)

warn Function

The “warn” function is Perl, is used to display a warning message without terminating the program.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

# die Function
say "To open an existing file, please provide the input file name as error_handling.txt.\nTo try out the \"die\" function, please provide the input file name as non_existent_file.txt.";
print "\nPlease provide the name of the input file: ";
my $dieInputFile = <STDIN>;
chomp $dieInputFile;

say "";

# Open the file in read mode
open(my $fh, '<', $dieInputFile) or die "ERROR! Could not open file: $!";
say "Please find the file contents below: ";
say "";

# Read the file line by line
while (my $line = <$fh>) {
    chomp $line;
    say $line;
}

say "";
say "";
say "";

# warn Function
say "To open an existing file, please provide the input file name as error_handling.txt.\nTo try out the \"warn\" function, please provide the input file name as too_large_file.txt.";
print "\nPlease provide the name of the input file: ";
my $warnInputFile = <STDIN>;
chomp $warnInputFile;

say "";

# Open the file in read mode
open($fh, '<', $warnInputFile) or warn "WARNING! Could not open file: $!";
say "Please find the file contents below: ";
say "";

# Read the file line by line
while (my $line = <$fh>) {
    chomp $line;
    say $line;
}
Output when the input is a file that exists (error_handling.txt)
Output when the input is a file that does not exist (too_large_file.txt)

eval Block

The “eval” block in Perl is used for exception handling. It allows us to capture and handle exceptions, preventing the program from terminating abruptly.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

# die Function
say "To open an existing file, please provide the input file name as error_handling.txt.\nTo try out the \"die\" function, please provide the input file name as non_existent_file.txt.";
print "\nPlease provide the name of the input file: ";
my $dieInputFile = <STDIN>;
chomp $dieInputFile;

say "";

# Open the file in read mode
open(my $fh, '<', $dieInputFile) or die "ERROR! Could not open file: $!";
say "Please find the file contents below: ";
say "";

# Read the file line by line
while (my $line = <$fh>) {
    chomp $line;
    say $line;
}

say "";
say "";
say "";

# warn Function
say "To open an existing file, please provide the input file name as error_handling.txt.\nTo try out the \"warn\" function, please provide the input file name as too_large_file.txt.";
print "\nPlease provide the name of the input file: ";
my $warnInputFile = <STDIN>;
chomp $warnInputFile;

say "";

# Open the file in read mode
open($fh, '<', $warnInputFile) or warn "WARNING! Could not open file: $!";
say "Please find the file contents below: ";
say "";

# Read the file line by line
while (my $line = <$fh>) {
    chomp $line;
    say $line;
}

say "";
say "";
say "";

# eval Block
say "To open an existing file, please provide the input file name as error_handling.txt.\nTo try out the \"eval\" block, please provide the input file name as corrupted_file.txt.";
print "\nPlease provide the name of the input file: ";
my $evalInputFile = <STDIN>;
chomp $evalInputFile;

say "";

# Open the file in read mode
eval {
    open($fh, '<', $evalInputFile) or die "ERROR! Could not open file: $!";
};
if ($@) {
    say "ERROR! Could not open file: $@";
}
say "Please find the file contents below: ";
say "";

# Read the file line by line
while (my $line = <$fh>) {
    chomp $line;
    say $line;
}
Output when the input is a file that exists (error_handling.txt)
Output when the input is a file that does not exist (corrupted_file.txt)

Try::Tiny Module

The Try::Tiny module in Perl can be used for more advanced error handling. It provides a simple and flexible way to handle exceptions.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";
use Try::Tiny;

# die Function
say "To open an existing file, please provide the input file name as error_handling.txt.\nTo try out the \"die\" function, please provide the input file name as non_existent_file.txt.";
print "\nPlease provide the name of the input file: ";
my $dieInputFile = <STDIN>;
chomp $dieInputFile;

say "";

# Open the file in read mode
open(my $fh, '<', $dieInputFile) or die "ERROR! Could not open file: $!";
say "Please find the file contents below: ";
say "";

# Read the file line by line
while (my $line = <$fh>) {
    chomp $line;
    say $line;
}

say "";
say "";
say "";

# warn Function
say "To open an existing file, please provide the input file name as error_handling.txt.\nTo try out the \"warn\" function, please provide the input file name as too_large_file.txt.";
print "\nPlease provide the name of the input file: ";
my $warnInputFile = <STDIN>;
chomp $warnInputFile;

say "";

# Open the file in read mode
open($fh, '<', $warnInputFile) or warn "WARNING! Could not open file: $!";
say "Please find the file contents below: ";
say "";

# Read the file line by line
while (my $line = <$fh>) {
    chomp $line;
    say $line;
}

say "";
say "";
say "";

# eval Block
say "To open an existing file, please provide the input file name as error_handling.txt.\nTo try out the \"eval\" block, please provide the input file name as corrupted_file.txt.";
print "\nPlease provide the name of the input file: ";
my $evalInputFile = <STDIN>;
chomp $evalInputFile;

say "";

# Open the file in read mode
eval {
    open($fh, '<', $evalInputFile) or die "ERROR! Could not open file: $!";
};
if ($@) {
    say "ERROR! Could not open file: $@";
}
say "Please find the file contents below: ";
say "";

# Read the file line by line
while (my $line = <$fh>) {
    chomp $line;
    say $line;
}

say "";
say "";
say "";

# Try::Tiny Module
say "To open an existing file, please provide the input file name as error_handling.txt.\nTo try out the \"Try::Tiny\" module, please provide the input file name as missing_file.txt.";
print "\nPlease provide the name of the input file: ";
my $tryTinyInputFile = <STDIN>;
chomp $tryTinyInputFile;

say "";

# Open the file in read mode
try {
    open($fh, '<', $tryTinyInputFile) or die "ERROR! Could not open file: $!";
} catch {
    warn "ERROR! Could not open file: $_\n";
};
say "Please find the file contents below: ";
say "";

# Read the file line by line
while (my $line = <$fh>) {
    chomp $line;
    say $line;
}
Output when the input is a file that exists (error_handling.txt)
Output when the input is a file that does not exist (missing_file.txt)

You can access the source code at: Perl/Perl-5 at master · Himashi-Karunathilake/Perl (github.com).

Link to Part 6: Perl — 6. In this article let’s look at using our… | by Himashi Karunathilake | Aug, 2023 | Medium

Perl
Introduction
Programming
Beginner
Scripting

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Himashi Karunathilake
Himashi Karunathilake

Written by Himashi Karunathilake

8 Followers
1 Following

I am a cybersecurity enthusiast and writer with a passion for demystifying complex topics. Join me as I explore the ever-evolving world of cybersecurity!

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