Perl Basic Syntax

Hello World Program

Let's start with the classic Hello World program:

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

print "Hello, World!\n";

Code explanation:

• #!/usr/bin/perl: Called shebang, tells the system which interpreter to use to execute the script
• use strict;: Enables strict mode, requiring variable declarations
• use warnings;: Enables warnings to help discover potential issues
• print: Output function
• \n: Newline character

Run the program:

chmod +x hello.pl
./hello.pl

Perl Program Structure

Basic Structure

A complete Perl program consists of the following parts:

#!/usr/bin/perl          # Shebang line (optional but recommended)

# Pragma (compile directives)
use strict;
use warnings;
use v5.38;               # Specify Perl version

# Main program code
# ... your code ...

1;                       # Return true value (required in modules)

Comments

Perl uses the # symbol for comments:

# This is a single-line comment

use strict;  # End-of-line comments also work

=begin comment
This is a multi-line comment
that can span multiple lines
=end comment

=for comment
This is also another way
to write multi-line comments
=cut

Perl Statements

Statement Separator

Perl uses semicolon ; as a statement separator:

print "First statement\n";
print "Second statement\n";

Note: The semicolon after the last statement can be omitted, but for clarity, it's recommended to always use a semicolon.

Statement Blocks

Statement blocks are enclosed in curly braces {}:

{
    print "Block start\n";
    print "Code inside block\n";
    print "Block end\n";
}

Output Statements

print Function

print "Hello\n";           # Basic output
print "Hello", " ", "World", "\n";  # Multiple arguments

# Without newline
print "Hello ";            # Output: Hello World
print "World\n";

say Function (Perl 5.10+)

The say function automatically adds a newline at the end:

use v5.10;  # or higher version

say "Hello";      # Automatically adds \n
say "World";      # Each output on a new line

printf Function

Formatted output:

my $name = "Alice";
my $age = 25;

printf "Name: %s, Age: %d\n", $name, $age;
# Output: Name: Alice, Age: 25

# Format numbers
printf "Pi: %.2f\n", 3.14159;  # Output: Pi: 3.14

printf "Decimal: %d, Hex: %x, Octal: %o\n", 255, 255, 255;
# Output: Decimal: 255, Hex: ff, Octal: 377

Input Statements

Reading from Standard Input

print "Enter your name: ";
my $name = <STDIN>;
chomp $name;               # Remove trailing newline
print "Hello, $name!\n";

Simplified Writing

print "Enter your name: ";
chomp(my $name = <STDIN>);
print "Hello, $name!\n";

Reading One Line

my $line = <STDIN>;    # Read one line (including newline)
my $line2 = <>;        # Shorthand, equivalent to <STDIN>

Variable Declarations

Using strict and warnings

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

# Variables must be declared
my $name = "Alice";
print "$name\n";

# Undeclared variables will cause an error
# print $age;  # Error: Global symbol "$age" requires explicit package name

my Declares Local Variables

my $name = "Bob";      # Declare and initialize
my $age;               # Declare without initializing (value is undef)
$age = 30;             # Assign value

print "$name is $age years old\n";

our Declares Global Variables

our $global_var = "global";

local Temporarily Modifies Variables

our $var = "original";

{
    local $var = "modified";  # Temporarily modify
    print "Inside block: $var\n";  # Output: modified
}

print "Outside block: $var\n";      # Output: original

Code Style Recommendations

Indentation

Use 4 spaces or 1 tab for indentation:

if ($condition) {
    print "Condition is true\n";
    if ($another_condition) {
        print "Nested condition\n";
    }
}

Variable Naming

# Scalars: lowercase letters, words separated by underscores
my $user_name = "Alice";
my $userAge = 25;         # CamelCase is also acceptable

# Arrays: plural form or add suffix
my @users = ("Alice", "Bob");
my @user_list = ("Alice", "Bob");

# Hashes: plural form or add suffix
my %user_data = (name => "Alice", age => 25);
my %scores = (math => 90, english => 85);

Parentheses Usage

# Function calls can omit parentheses
print "Hello\n";
print("Hello\n");        # Can also use parentheses

# Conditional statements recommend using parentheses
if ($condition) {
    # ...
}

Compilation and Execution

Direct Execution

perl script.pl

Execute as Script

chmod +x script.pl
./script.pl

Check Syntax (No Execution)

perl -c script.pl

Enable Warnings

perl -w script.pl

Debug Mode

perl -d script.pl

Practice Examples

Example 1: Simple Calculator

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

print "Enter first number: ";
chomp(my $num1 = <STDIN>);

print "Enter second number: ";
chomp(my $num2 = <STDIN>);

print "Choose operation (+, -, *, /): ";
chomp(my $op = <STDIN>);

my $result;

if ($op eq '+') {
    $result = $num1 + $num2;
} elsif ($op eq '-') {
    $result = $num1 - $num2;
} elsif ($op eq '*') {
    $result = $num1 * $num2;
} elsif ($op eq '/') {
    $result = $num1 / $num2;
} else {
    print "Invalid operator\n";
    exit;
}

printf "Result: %.2f\n", $result;

Example 2: User Information Collection

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

print "=== User Information Collection ===\n";

print "Name: ";
chomp(my $name = <STDIN>);

print "Age: ";
chomp(my $age = <STDIN>);

print "City: ";
chomp(my $city = <STDIN>);

print "\n=== User Information ===\n";
printf "Name: %s\n", $name;
printf "Age: %d\n", $age;
printf "City: %s\n", $city;

if ($age >= 18) {
    print "Status: Adult\n";
} else {
    print "Status: Minor\n";
}

Example 3: Multi-line Input Processing

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

print "Enter multiple lines (Ctrl+D to end):\n";
my @lines = <STDIN>;    # Read all lines into an array

print "\nYou entered " . scalar(@lines) . " lines:\n";
print @lines;          # Output all lines

Common Errors

1. Forgetting Semicolon

print "Hello\n"    # Error: missing semicolon
print "World\n";   # Syntax error

2. Undeclared Variables

use strict;
print $name;       # Error: variable must be declared

3. Confusing Scalars and Arrays

my @numbers = (1, 2, 3);
print $numbers;    # Error: should use @numbers or $numbers[0]

4. Typos

my $name = "Alice";
print "$nmae\n";   # Error: variable name typo

Summary

In this chapter, we learned Perl's basic syntax:

1. ✅ Program structure and comments
2. ✅ Input and output statements
3. ✅ Variable declarations
4. ✅ Code style guidelines
5. ✅ Compilation and execution methods

Next, we will learn Perl Data Types.