Perl Conditional Statements

if Statement

Basic if Statement

perl

my $age = 18;

if ($age >= 18) {
    print "You are an adult\n";
}

if-else Statement

perl

my $age = 15;

if ($age >= 18) {
    print "You are an adult\n";
} else {
    print "You are a minor\n";
}

if-elsif-else Statement

perl

my $score = 85;

if ($score >= 90) {
    print "Excellent\n";
} elsif ($score >= 80) {
    print "Good\n";
} elsif ($score >= 70) {
    print "Average\n";
} elsif ($score >= 60) {
    print "Pass\n";
} else {
    print "Fail\n";
}

unless Statement

unless is shorthand for if not, executes the block when condition is false.

Basic unless Statement

perl

my $is_raining = 0;

unless ($is_raining) {
    print "We can go out\n";
}

unless-else Statement

perl

my $has_ticket = 0;

unless ($has_ticket) {
    print "Please buy a ticket first\n";
} else {
    print "Please enter\n";
}

Nested Usage

perl

my $weather = "sunny";
my $has_money = 1;

unless ($weather eq "rainy") {
    print "Weather is good\n";
    unless ($has_money) {
        print "But no money\n";
    } else {
        print "Can go shopping\n";
    }
}

Ternary Operator

Basic Usage

perl

my $age = 18;
my $status = $age >= 18 ? "Adult" : "Minor";
print $status;  # Adult

Nested Ternary Operator

perl

my $score = 75;
my $grade = $score >= 90 ? "A" :
            $score >= 80 ? "B" :
            $score >= 70 ? "C" :
            $score >= 60 ? "D" : "F";
print $grade;  # C

Assignment and Function Calls

perl

my $user = defined($input_user) ? $input_user : "Guest";
print defined($file) ? $file : "default.txt";

given-when Statement (Perl 5.10+)

Basic given-when

perl

use v5.10;

my $day = "Monday";

given ($day) {
    when ("Monday")    { print "Monday\n"; }
    when ("Tuesday")   { print "Tuesday\n"; }
    when ("Wednesday") { print "Wednesday\n"; }
    when ("Thursday")  { print "Thursday\n"; }
    when ("Friday")    { print "Friday\n"; }
    default            { print "Weekend\n"; }
}

Using Regular Expressions

perl

use v5.10;

my $input = "hello world";

given ($input) {
    when (/^hello/) { print "Starts with hello\n"; }
    when (/world$/) { print "Ends with world\n"; }
    when (/hello/)  { print "Contains hello\n"; }
    default        { print "Other\n"; }
}

Using Multiple Conditions

perl

use v5.10;

my $number = 5;

given ($number) {
    when ([1, 2, 3])    { print "1, 2, or 3\n"; }
    when (4)            { print "4\n"; }
    when ([5, 6, 7])    { print "5, 6, or 7\n"; }
    default             { print "Other number\n"; }
}

Using continue

perl

use v5.10;

my $day = "Wednesday";

given ($day) {
    when ("Monday")    { print "Weekday\n"; continue; }
    when ("Tuesday")   { print "Weekday\n"; continue; }
    when ("Wednesday") { print "Weekday\n"; continue; }
    when ("Thursday")  { print "Weekday\n"; continue; }
    when ("Friday")    { print "Weekday\n"; continue; }
}
print "Continue execution\n";

Logical Operators and Conditions

Using && and ||

perl

my $age = 25;
my $has_license = 1;

# Logical AND
if ($age >= 18 && $has_license) {
    print "Can drive\n";
}

# Logical OR
my $weather = "sunny";
my $has_umbrella = 0;

if ($weather eq "sunny" || $has_umbrella) {
    print "Can go out\n";
}

Using and and or

perl

# Low-precedence logical operators
if ($age >= 18 and $has_license) {
    print "Can drive\n";
}

if ($weather eq "sunny" or $has_umbrella) {
    print "Can go out\n";
}

Short-circuit Evaluation

perl

# If first condition is true, second won't execute
my $x = 1;
my $y = 0;

if ($x || ($y = 10)) {
    print "Condition is true\n";
}
print "$y\n";  # $y is still 0

# If first condition is false, second won't execute
my $a = 0;
my $b = 0;

if ($a && ($b = 10)) {
    print "Condition is true\n";
}
print "$b\n";  # $b is still 0

String Comparison

String Equality and Inequality

perl

my $str1 = "hello";
my $str2 = "world";

if ($str1 eq $str2) {
    print "Strings are equal\n";
} else {
    print "Strings are not equal\n";
}

unless ($str1 eq $str2) {
    print "Strings are not equal\n";
}

String Magnitude Comparison

perl

my $str1 = "apple";
my $str2 = "banana";

if ($str1 lt $str2) {
    print "'$str1' comes before '$str2' in dictionary order\n";
}

if ($str1 le $str2) {
    print "'$str1' comes before or is equal to '$str2' in dictionary order\n";
}

Defined Check

Using defined

perl

my $value;

if (defined($value)) {
    print "Value is defined\n";
} else {
    print "Value is undefined\n";
}

my $num = 42;
if (defined($num)) {
    print "Numeric value: $num\n";
}

Using Defined-or Operator

perl

my $input;

# If $input is undefined, use default value
my $value = $input // "default";
print $value;  # default

$input = "user input";
$value = $input // "default";
print $value;  # user input

Using exists to Check Hash Keys

perl

my %hash = (name => "Alice", age => 25);

if (exists $hash{name}) {
    print "Key 'name' exists\n";
}

unless (exists $hash{city}) {
    print "Key 'city' does not exist\n";
}

Complex Conditions

Combining Multiple Conditions

perl

my $age = 25;
my $has_job = 1;
my $has_car = 0;
my $has_license = 1;

if ($age >= 18 && $has_license && ($has_car || $has_job)) {
    print "Meets conditions\n";
} else {
    print "Does not meet conditions\n";
}

Using Parentheses for Clarity

perl

# Clear condition combination
if (($age >= 18 and $has_license) or ($age >= 21 and $has_job)) {
    print "Can apply\n";
}

De Morgan's Law

perl

# Original condition
if ($age < 18 or $income < 1000) {
    print "Not eligible\n";
}

# Using De Morgan's law
unless ($age >= 18 and $income >= 1000) {
    print "Not eligible\n";
}

Practice Examples

Example 1: Grade System

perl

#!/usr/bin/perl
use strict;
use warnings;

print "Enter score (0-100): ";
chomp(my $score = <STDIN>);

if ($score < 0 || $score > 100) {
    print "Invalid score\n";
} elsif ($score >= 90) {
    print "A (Excellent)\n";
} elsif ($score >= 80) {
    print "B (Good)\n";
} elsif ($score >= 70) {
    print "C (Average)\n";
} elsif ($score >= 60) {
    print "D (Pass)\n";
} else {
    print "F (Fail)\n";
}

perl

#!/usr/bin/perl
use strict;
use warnings;

my %users = (
    "alice" => "password123",
    "bob" => "secure456",
    "charlie" => "mypass789"
);

print "Username: ";
chomp(my $username = <STDIN>);

print "Password: ";
chomp(my $password = <STDIN>);

if (exists $users{$username} && $users{$username} eq $password) {
    print "Login successful! Welcome, $username!\n";
} elsif (!exists $users{$username}) {
    print "User does not exist\n";
} else {
    print "Incorrect password\n";
}

perl

#!/usr/bin/perl
use strict;
use warnings;
use v5.10;

print "=== Menu ===\n";
print "1. Display Info\n";
print "2. Calculator\n";
print "3. Exit\n";

print "Choose (1-3): ";
chomp(my $choice = <STDIN>);

given ($choice) {
    when ("1") {
        print "=== Info ===\n";
        print "Version: 1.0\n";
        print "Author: Your Name\n";
    }
    when ("2") {
        print "=== Calculator ===\n";
        print "Enter first number: ";
        chomp(my $a = <STDIN>);
        print "Enter second number: ";
        chomp(my $b = <STDIN>);
        print "Sum: " . ($a + $b) . "\n";
    }
    when ("3") {
        print "Goodbye!\n";
        exit;
    }
    default {
        print "Invalid choice\n";
    }
}

Example 4: Leap Year Check

perl

#!/usr/bin/perl
use strict;
use warnings;

print "Enter year: ";
chomp(my $year = <STDIN>);

my $is_leap = 0;

if ($year % 4 == 0) {
    if ($year % 100 == 0) {
        if ($year % 400 == 0) {
            $is_leap = 1;
        }
    } else {
        $is_leap = 1;
    }
}

if ($is_leap) {
    print "$year is a leap year\n";
} else {
    print "$year is not a leap year\n";
}

# Simplified version
my $is_leap_simple = ($year % 4 == 0 and $year % 100 != 0) or ($year % 400 == 0);
print "Simplified check: $is_leap_simple\n";

Summary

In this chapter, we learned Perl conditional statements:

✅ if/elsif/else statements
✅ unless statements
✅ Ternary operator
✅ given-when statements
✅ Logical operators and conditions
✅ String comparison
✅ Defined checks
✅ Complex condition combinations

Next, we will learn Perl Loops.

Perl Conditional Statements ​

if Statement ​

Basic if Statement ​

if-else Statement ​

if-elsif-else Statement ​

unless Statement ​

Basic unless Statement ​

unless-else Statement ​

Nested Usage ​

Ternary Operator ​

Basic Usage ​

Nested Ternary Operator ​

Assignment and Function Calls ​

given-when Statement (Perl 5.10+) ​

Basic given-when ​

Using Regular Expressions ​

Using Multiple Conditions ​

Using continue ​

Logical Operators and Conditions ​

Using && and || ​

Using and and or ​

Short-circuit Evaluation ​

String Comparison ​

String Equality and Inequality ​

String Magnitude Comparison ​

Defined Check ​

Using defined ​

Using Defined-or Operator ​

Using exists to Check Hash Keys ​

Complex Conditions ​

Combining Multiple Conditions ​

Using Parentheses for Clarity ​

De Morgan's Law ​

Practice Examples ​

Example 1: Grade System ​

Example 2: Login Verification ​

Example 3: Menu Selection System ​

Example 4: Leap Year Check ​

Summary ​

Perl Conditional Statements

if Statement

Basic if Statement

if-else Statement

if-elsif-else Statement

unless Statement

Basic unless Statement

unless-else Statement

Nested Usage

Ternary Operator

Basic Usage

Nested Ternary Operator

Assignment and Function Calls

given-when Statement (Perl 5.10+)

Basic given-when

Using Regular Expressions

Using Multiple Conditions

Using continue

Logical Operators and Conditions

Using && and ||

Using and and or

Short-circuit Evaluation

String Comparison

String Equality and Inequality

String Magnitude Comparison

Defined Check

Using defined

Using Defined-or Operator

Using exists to Check Hash Keys

Complex Conditions

Combining Multiple Conditions

Using Parentheses for Clarity

De Morgan's Law

Practice Examples

Example 1: Grade System

Example 2: Login Verification

Example 3: Menu Selection System

Example 4: Leap Year Check

Summary