Basics

Hello World

#!/usr/bin/perl
use strict;
use warnings;
print "Hello, World!\n";
say "Hello, World!";  # with use feature 'say';

Run Perl

perl script.pl           # run a file
perl -e 'print "hi\n"'  # run inline
perl -ne 'print' file   # process file line by line

Comments & Documentation

# single-line comment
=pod
Multi-line POD documentation
=cut

Variables

Sigils

$scalar	Single value (string, number, reference)
@array	Ordered list of scalars
%hash	Key-value pairs
$array[0]	Access single array element (scalar context)
$hash{key}	Access single hash value (scalar context)

Scalar Variables

my $name = "Perl";        # string
my $version = 5.40;       # number
my $count = 42;           # integer
my $undef;                # undefined (undef)
my $combined = "$name v$version";  # interpolation

Context

my @arr = (1, 2, 3);
my $count = @arr;          # scalar context: 3
my @copy = @arr;           # list context: (1, 2, 3)
my $len = scalar @arr;     # force scalar context

Special Variables

$_	Default variable (topic)
@_	Subroutine arguments
$!	System error message
$@	Error from eval
$0	Program name
@ARGV	Command-line arguments
%ENV	Environment variables

Operators

Comparison Operators

==, !=, <, >, <=, >=	Numeric comparison
eq, ne, lt, gt, le, ge	String comparison
<=>	Numeric spaceship (returns -1, 0, 1)
cmp	String spaceship
=~	Regex match / bind
!~	Regex negated match

String Operators

my $full = "Hello" . " " . "World";  # concatenation
my $line = "-" x 40;                  # repetition
my $len = length($full);              # 11

Logical Operators

&& / and	Logical AND (low precedence: `and`)
\|\| / or	Logical OR (low precedence: `or`)
//	Defined-or (returns left if defined)
! / not	Logical NOT
? :	Ternary conditional

Control Flow

Conditionals

if ($x > 0) { print "positive\n"; }
elsif ($x == 0) { print "zero\n"; }
else { print "negative\n"; }
print "yes\n" if $condition;     # postfix if
print "no\n" unless $condition;  # postfix unless

Loops

for my $i (0..9) { print "$i\n"; }
foreach my $item (@array) { print "$item\n"; }
while ($line = ) { chomp $line; }
until ($done) { last if check(); }

Loop Control

next	Skip to next iteration (like `continue`)
last	Exit loop (like `break`)
redo	Restart current iteration
next LABEL	Skip to next iteration of labeled loop
last LABEL	Exit labeled loop

Given / When

use feature 'switch';
given ($status) {
    when ("ok")    { say "success"; }
    when ("error") { say "failed"; }
    default        { say "unknown"; }
}

Subroutines

Basic Subroutine

sub greet {
    my ($name) = @_;
    return "Hello, $name!";
}
my $msg = greet("Alice");

Default & Named Parameters

sub connect {
    my (%opts) = @_;
    my $host = $opts{host} // "localhost";
    my $port = $opts{port} // 5432;
    return "$host:$port";
}
connect(host => "db.example.com", port => 3306);

Subroutine References

my $double = sub { return $_[0] * 2; };
print $double->(5);   # 10
my @sorted = sort { $a <=> $b } @nums;

Prototypes & Signatures

use feature 'signatures';
sub add($a, $b) { return $a + $b; }
sub greet($name, $greeting = "Hello") {
    return "$greeting, $name!";
}

Regex

Matching

if ($str =~ /pattern/) { print "matched\n"; }
if ($str =~ /(\d+)/) { print "number: $1\n"; }
my @matches = ($str =~ /(\w+)/g);  # all matches

Substitution

$str =~ s/old/new/;      # first occurrence
$str =~ s/old/new/g;     # global (all occurrences)
$str =~ s/^\s+|\s+$//g;  # trim whitespace
(my $clean = $str) =~ s/\W//g;  # non-destructive copy

Modifiers

/i	Case-insensitive
/g	Global (all matches)
/m	Multi-line (`^` and `$` match line boundaries)
/s	Single-line (`.` matches newline)
/x	Extended (allow whitespace and comments)

Common Patterns

\d, \D	Digit / non-digit
\w, \W	Word char / non-word char
\s, \S	Whitespace / non-whitespace
\b	Word boundary
(?: ... )	Non-capturing group
(?<name> ... )	Named capture (access via `$+{name}`)

File I/O

Open & Read

open(my $fh, '<', 'data.txt') or die "Cannot open: $!";
while (my $line = <$fh>) {
    chomp $line;
    print "$line\n";
}
close($fh);

Write & Append

open(my $fh, '>', 'out.txt') or die "Cannot open: $!";
print $fh "Hello\n";
close($fh);
open(my $fh, '>>', 'log.txt') or die "Cannot open: $!";
print $fh "entry\n";
close($fh);

Slurp Entire File

use File::Slurp;
my $content = read_file('data.txt');
my @lines = read_file('data.txt', chomp => 1);

File Tests

-e $path	File exists
-f $path	Is a regular file
-d $path	Is a directory
-r / -w / -x	Readable / writable / executable
-s $path	File size in bytes (0 if empty)
-z $path	File is zero size

Arrays & Hashes

Arrays

my @arr = (1, 2, 3, 4, 5);
push @arr, 6;              # append
my $last = pop @arr;       # remove last
my $first = shift @arr;    # remove first
unshift @arr, 0;           # prepend
my @slice = @arr[1..3];    # slice

Array Functions

scalar @arr	Number of elements
push / pop	Add/remove from end
shift / unshift	Remove/add from beginning
splice(@a, 2, 1)	Remove 1 element at index 2
sort @arr	Sort alphabetically
reverse @arr	Reverse order
grep { /pat/ } @arr	Filter by pattern
map { $_ * 2 } @arr	Transform each element
join(',', @arr)	Join into string

Hashes

my %user = (name => "Alice", age => 30);
$user{email} = "a\@b.com";    # add pair
delete $user{age};             # remove pair
my @keys = keys %user;
my @vals = values %user;

Hash Iteration

while (my ($k, $v) = each %hash) {
    print "$k => $v\n";
}
for my $key (sort keys %hash) {
    print "$key: $hash{$key}\n";
}

References

Creating References

my $scalar_ref = \$name;
my $array_ref  = \@arr;
my $hash_ref   = \%hash;
my $anon_arr   = [1, 2, 3];       # anonymous array ref
my $anon_hash  = {a => 1, b => 2}; # anonymous hash ref

Dereferencing

print $$scalar_ref;          # dereference scalar
print $array_ref->[0];       # arrow notation
print $hash_ref->{key};      # arrow notation
my @copy = @$array_ref;      # dereference to array
my %copy = %$hash_ref;       # dereference to hash

Complex Data Structures

my @users = (
    { name => "Alice", age => 30 },
    { name => "Bob",   age => 25 },
);
print $users[0]->{name};   # "Alice"

ref() Function

ref($r) eq 'SCALAR'	Reference to scalar
ref($r) eq 'ARRAY'	Reference to array
ref($r) eq 'HASH'	Reference to hash
ref($r) eq 'CODE'	Reference to subroutine

Modules

Using Modules

use strict;
use warnings;
use List::Util qw(sum max min);
use File::Basename;
use Cwd qw(abs_path);

Creating a Module

# MyModule.pm
package MyModule;
use Exporter 'import';
our @EXPORT_OK = qw(helper);
sub helper { return "help"; }
1;  # module must return true

Common Core Modules

List::Util	sum, max, min, reduce, any, all
File::Basename	basename, dirname, fileparse
File::Path	make_path, remove_tree
Getopt::Long	Command-line option parsing
JSON	encode_json, decode_json
LWP::Simple	get($url) — simple HTTP client
Data::Dumper	Debug dump of data structures
Carp	croak, confess — better error messages

CPAN

cpan install Module::Name        # install from CPAN
cpanm Module::Name               # cpanminus (faster)
perldoc Module::Name             # read module docs

PERL QUICK REFERENCE