PERLOOTUT(1perl)

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version 5.44.0 · 2 versions
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💖 PERLOOTUT 💖

okay bestie, this is PERLOOTUT(1perl) from the Perl Programmers Reference Guide, and it is literally your ✨object-oriented Perl✨ tutorial. Grab a snack, we're doing this. 🍓

🎀 NAME 🎀

perlootut — Object-Oriented Programming in Perl Tutorial

🗓️ DATE 🗓️

So this doc? Born February, 2011, and its last major glow-up was February, 2013.

If you're reading this in the future then lowkey the state of the art might have moved on. The doc itself says: start by reading the perlootut document in the latest stable release of Perl, not this version. fr fr. 💅

🌸 DESCRIPTION 🌸

This doc is your intro to object-oriented programming in Perl. It kicks off with a quick lil overview of the concepts behind object-oriented design, and then introduces a bunch of different OO systems from CPAN https://www.cpan.org that build on top of what Perl already gives you.

By default, Perl's built-in OO system is very minimal — it leaves most of the work to you. And honestly? That minimalism made a lot of sense in 1994, but in the years since Perl 5.0 we've watched a bunch of common patterns emerge in Perl OO. Lucky for us, Perl's flexibility let a whole rich ecosystem of Perl OO systems flourish. 🌈

If you wanna know how Perl OO works under the hood, the perlobj document has all the nitty gritty details. 👑

Heads up bestie: this doc assumes you already get the basics of Perl syntax, variable types, operators, and subroutine calls. If you don't yet, please read perlintro first. You should also read the perlsyn, perlop, and perlsub documents.

🦋 OBJECT-ORIENTED FUNDAMENTALS 🦋

Most object systems share a bunch of common concepts. You've probably heard terms like "class", "object", "method", and "attribute" before. Understanding these concepts makes it so much easier to read and write object-oriented code. And if you're already familiar with them? Still skim this section, bestie, because it explains each concept in terms of Perl's OO implementation specifically. 💜

Perl's OO system is class-based. Class-based OO is pretty common — it's used by Java, C++, C#, Python, Ruby, and many other languages. There are other object orientation paradigms too, though. JavaScript is the most popular language to use another paradigm: its OO system is prototype-based.

🌟 Object 🌟

An object is a data structure that bundles together data and subroutines which operate on that data. An object's data is called attributes, and its subroutines are called methods. You can think of an object as a noun (a person, a web service, a computer).

An object represents a single discrete thing. For example, an object might represent a file. The attributes for a file object might include its path, content, and last modification time. If we created an object to represent /etc/hostname on a machine named "foo.example.com", that object's path would be "/etc/hostname", its content would be "foo\n", and it's last modification time would be 1304974868 seconds since the beginning of the epoch.

The methods associated with a file might include rename() and write().

In Perl most objects are hashes, but the OO systems we recommend keep you from having to worry about that. In practice, it's best to consider an object's internal data structure opaque. 🙈

💫 Class 💫

A class defines the behavior of a category of objects. A class is a name for a category (like "File"), and a class also defines the behavior of objects in that category.

All objects belong to a specific class. For example, our /etc/hostname object belongs to the "File" class. When we want to create a specific object, we start with its class, and construct or instantiate an object. A specific object is often referred to as an instance of a class.

In Perl, any package can be a class. The difference between a package which is a class and one which isn't is based on how the package is used. Here's our "class declaration" for the "File" class:

package File;

In Perl, there is no special keyword for constructing an object. However, most OO modules on CPAN use a method named new() to construct a new object:

my $hostname = File->new(
    path          => '/etc/hostname',
    content       => "foo\n",
    last_mod_time => 1304974868,
);

(Don't worry about that -> operator, it will be explained later. 😌)

🧚 Blessing 🧚

As we said earlier, most Perl objects are hashes, but an object can be an instance of any Perl data type (scalar, array, etc.). Turning a plain data structure into an object is done by blessing that data structure using Perl's bless function.

While we strongly suggest you don't build your objects from scratch, you should know the term bless. A blessed data structure (aka "a referent") is an object. We sometimes say that an object has been "blessed into a class".

Once a referent has been blessed, the blessed function from the Scalar::Util core module can tell us its class name. This subroutine returns an object's class when passed an object, and false otherwise.

use Scalar::Util 'blessed';

print blessed($hash);      # undef
print blessed($hostname);  # File

🎀 Constructor 🎀

A constructor creates a new object. In Perl, a class's constructor is just another method — unlike some other languages, which provide syntax for constructors. Most Perl classes use "new" as the name for their constructor:

my $file = File->new(...);

💅 Methods 💅

You already learned that a method is a subroutine that operates on an object. You can think of a method as the things that an object can do. If an object is a noun, then methods are its verbs (save, print, open).

In Perl, methods are simply subroutines that live in a class's package. Methods are always written to receive the object as their first argument:

sub print_info {
    my $self = shift;

    print "This file is at ", $self->path, "\n";
}

$file->print_info;
# The file is at /etc/hostname

What makes a method special is how it's called. The arrow operator (->) tells Perl that we are calling a method.

When we make a method call, Perl arranges for the method's invocant to be passed as the first argument. Invocant is a fancy name for the thing on the left side of the arrow. The invocant can either be a class name or an object. We can also pass additional arguments to the method:

sub print_info {
    my $self   = shift;
    my $prefix = shift // "This file is at ";

    print $prefix, ", ", $self->path, "\n";
}

$file->print_info("The file is located at ");
# The file is located at /etc/hostname

🍓 Attributes 🍓

Each class can define its attributes. When we instantiate an object, we assign values to those attributes. For example, every "File" object has a path. Attributes are sometimes called properties.

Perl has no special syntax for attributes. Under the hood, attributes are often stored as keys in the object's underlying hash, but don't worry about this.

We recommend that you only access attributes via accessor methods. These are methods that can get or set the value of each attribute. We saw this earlier in the print_info() example, which calls $self->path.

You might also see the terms getter and setter. These are two types of accessors. A getter gets the attribute's value, while a setter sets it. Another term for a setter is mutator.

Attributes are typically defined as read-only or read-write. Read-only attributes can only be set when the object is first created, while read-write attributes can be altered at any time.

The value of an attribute may itself be another object. For example, instead of returning its last mod time as a number, the "File" class could return a DateTime object representing that value.

It's possible to have a class that does not expose any publicly settable attributes. Not every class has attributes and methods.

🦄 Polymorphism 🦄

Polymorphism is a fancy way of saying that objects from two different classes share an API. For example, we could have "File" and "WebPage" classes which both have a print_content() method. This method might produce different output for each class, but they share a common interface.

While the two classes may differ in many ways, when it comes to the print_content() method, they are the same. This means that we can try to call the print_content() method on an object of either class, and we don't have to know what class the object belongs to! 🤯

Polymorphism is one of the key concepts of object-oriented design.

👑 Inheritance 👑

Inheritance lets you create a specialized version of an existing class. Inheritance lets the new class reuse the methods and attributes of another class.

For example, we could create a "File::MP3" class which inherits from "File". A "File::MP3" is-a more specific type of "File". All mp3 files are files, but not all files are mp3 files.

We often refer to inheritance relationships as parent-child or "superclass"/"subclass" relationships. Sometimes we say that the child has an is-a relationship with its parent class.

"File" is a superclass of "File::MP3", and "File::MP3" is a subclass of "File".

package File::MP3;

use parent 'File';

The parent module is one of several ways that Perl lets you define inheritance relationships.

Perl allows multiple inheritance, which means that a class can inherit from multiple parents. While this is possible, we strongly recommend against it. Generally, you can use roles to do everything you can do with multiple inheritance, but in a cleaner way. 💅

Note that there's nothing wrong with defining multiple subclasses of a given class. This is both common and safe. For example, we might define "File::MP3::FixedBitrate" and "File::MP3::VariableBitrate" classes to distinguish between different types of mp3 file.

🌺 Overriding methods and method resolution 🌺

Inheritance allows two classes to share code. By default, every method in the parent class is also available in the child. The child can explicitly override a parent's method to provide its own implementation. For example, if we have a "File::MP3" object, it has the print_info() method from "File":

my $cage = File::MP3->new(
    path          => 'mp3s/My-Body-Is-a-Cage.mp3',
    content       => $mp3_data,
    last_mod_time => 1304974868,
    title         => 'My Body Is a Cage',
);

$cage->print_info;
# The file is at mp3s/My-Body-Is-a-Cage.mp3

If we wanted to include the mp3's title in the greeting, we could override the method:

package File::MP3;

use parent 'File';

sub print_info {
    my $self = shift;

    print "This file is at ", $self->path, "\n";
    print "Its title is ", $self->title, "\n";
}

$cage->print_info;
# The file is at mp3s/My-Body-Is-a-Cage.mp3
# Its title is My Body Is a Cage

The process of determining what method should be used is called method resolution. What Perl does is look at the object's class first (File::MP3 in this case). If that class defines the method, then that class's version of the method is called. If not, Perl looks at each parent class in turn. For File::MP3, its only parent is File. If File::MP3 does not define the method, but File does, then Perl calls the method in File.

If File inherited from DataSource, which inherited from Thing, then Perl would keep looking "up the chain" if necessary.

It is possible to explicitly call a parent method from a child:

package File::MP3;

use parent 'File';

sub print_info {
    my $self = shift;

    $self->SUPER::print_info();
    print "Its title is ", $self->title, "\n";
}

The SUPER:: bit tells Perl to look for the print_info() in the File::MP3 class's inheritance chain. When it finds the parent class that implements this method, the method is called.

We mentioned multiple inheritance earlier. The main problem with multiple inheritance is that it greatly complicates method resolution. See perlobj for more details.

Encapsulation 💖

Encapsulation is the idea that an object is opaque. When another developer uses your class, they don't need to know how it is implemented, they just need to know what it does.

Encapsulation is important for several reasons. First, it allows you to separate the public API from the private implementation. This means you can change that implementation without breaking the API.

Second, when classes are well encapsulated, they become easier to subclass. Ideally, a subclass uses the same APIs to access object data that its parent class uses. In reality, subclassing sometimes involves violating encapsulation, but a good API can minimize the need to do this.

We mentioned earlier that most Perl objects are implemented as hashes under the hood. The principle of encapsulation tells us that we should not rely on this. Instead, we should use accessor methods to access the data in that hash. The object systems that we recommend below all automate the generation of accessor methods. If you use one of them, you should never have to access the object as a hash directly.

Composition 🦋

In object-oriented code, we often find that one object references another object. This is called composition, or a has-a relationship.

Earlier, we mentioned that the File class's last_mod_time accessor could return a DateTime object. This is a perfect example of composition. We could go even further, and make the path and content accessors return objects as well. The File class would then be composed of several other objects.

Roles 🎀

Roles are something that a class does, rather than something that it is. Roles are relatively new to Perl, but have become rather popular. Roles are applied to classes. Sometimes we say that classes consume roles.

Roles are an alternative to inheritance for providing polymorphism. Let's assume we have two classes, Radio and Computer. Both of these things have on/off switches. We want to model that in our class definitions.

We could have both classes inherit from a common parent, like Machine, but not all machines have on/off switches. We could create a parent class called HasOnOffSwitch, but that is very artificial. Radios and computers are not specializations of this parent. This parent is really a rather ridiculous creation.

This is where roles come in. It makes a lot of sense to create a HasOnOffSwitch role and apply it to both classes. This role would define a known API like providing turn_on() and turn_off() methods.

Perl does not have any built-in way to express roles. In the past, people just bit the bullet and used multiple inheritance. Nowadays, there are several good choices on CPAN for using roles.

When to Use OO 👑

Object Orientation is not the best solution to every problem. In Perl Best Practices (copyright 2004, Published by O'Reilly Media, Inc.), Damian Conway provides a list of criteria to use when deciding if OO is the right fit for your problem:

  • The system being designed is large, or is likely to become large.
  • The data can be aggregated into obvious structures, especially if there's a large amount of data in each aggregate.
  • The various types of data aggregate form a natural hierarchy that facilitates the use of inheritance and polymorphism.
  • You have a piece of data on which many different operations are applied.
  • You need to perform the same general operations on related types of data, but with slight variations depending on the specific type of data the operations are applied to.
  • It's likely you'll have to add new data types later.
  • The typical interactions between pieces of data are best represented by operators.
  • The implementation of individual components of the system is likely to change over time.
  • The system design is already object-oriented.
  • Large numbers of other programmers will be using your code modules.

💖 PERL OO SYSTEMS 💖

okay bestie, real talk. we already established that Perl's built-in OO system is super minimal, but also lowkey flexible. and over the years? so many people have built systems on top of it to give you more features and more convenience. iconic behavior fr.

and we strongly recommend you use one of these systems. like, even the most minimal one of them wipes out a TON of repetitive boilerplate. there's genuinely no good reason to write your classes from scratch in Perl. none. zero. don't do it. 🫶

if you're the kind of girl who wants to see the guts underneath all this, go check out perlobj. it's giving anatomy class. 💀

🦋 Moose 🦋

Moose calls itself a "postmodern object system for Perl 5". and bestie, do NOT be scared — the "postmodern" label is just a callback to Larry's description of Perl as "the first postmodern computer language". history! ✨

Moose gives you a complete, modern OO system. its biggest influence is the Common Lisp Object System, but it also borrows ideas from Smalltalk and several other languages. Moose was created by Stevan Little, and it draws heavily from his work on the Raku OO design. king behavior. 👑

here's our File class using Moose:

package File;
use Moose;

has path          => ( is => 'ro' );
has content       => ( is => 'ro' );
has last_mod_time => ( is => 'ro' );

sub print_info {
    my $self = shift;

    print "This file is at ", $self->path, "\n";
}

Moose provides a whole bunch of features:

  • Declarative sugar — Moose gives you a layer of declarative "sugar" for defining classes. and that sugar is just a set of exported functions that make declaring how your class works simpler and more palatable. the whole vibe is: you describe what your class is, instead of having to tell Perl how to implement it. the has() subroutine declares an attribute, and Moose automatically creates accessors for those attributes. it also takes care of creating a new() method for you. that constructor knows about the attributes you declared, so you can set them when creating a new File. slay.
  • Roles built-in — Moose lets you define roles the same way you define classes:
package HasOnOffSwitch;
use Moose::Role;

has is_on => (
    is  => 'rw',
    isa => 'Bool',
);

sub turn_on {
    my $self = shift;
    $self->is_on(1);
}

sub turn_off {
    my $self = shift;
    $self->is_on(0);
}
  • A miniature type system — see that example above? we passed isa => 'Bool' to has() when creating our is_on attribute. that tells Moose this attribute MUST be a boolean value. if we try to set it to an invalid value, our code will throw an error. protective. we love that. 🎀
  • Full introspection and manipulation — Perl's built-in introspection features are fairly minimal. Moose builds on top of them and creates a full introspection layer for your classes. so you can ask questions like "what methods does the File class implement?" and it also lets you modify your classes programmatically. nosy and powerful, the dream combo. 💅
  • Self-hosted and extensible — Moose describes itself using its own introspection API. besides being a cool trick, this means you can extend Moose using Moose itself. recursion but make it cute.
  • Rich ecosystem — there's a rich ecosystem of Moose extensions on CPAN under the MooseX namespace, at https://metacpan.org/search?q=MooseX. plus, so many modules on CPAN already use Moose, so you get lots of examples to learn from. free tutorials basically.
  • Many more features — Moose is a very powerful tool and we can't cover all of its features here. we encourage you to learn more by reading the Moose documentation, starting with Moose::Manual at https://metacpan.org/pod/Moose::Manual.

but of course, Moose isn't perfect. nothing is, bestie. 😭

Moose can make your code slower to load. Moose itself is not small, and it does a LOT of code generation when you define your class. that code generation means your runtime code is as fast as it can be — but you pay for it when your modules are first loaded.

this load time hit can be a problem when startup speed matters. like with a command-line script, or a "plain vanilla" CGI script that has to be loaded every single time it's executed. ouch.

before you panic though — know that lots of people DO use Moose for command-line tools and other startup-sensitive code. we encourage you to try Moose out first before worrying about startup speed. vibe check it yourself.

Moose also has several dependencies on other modules. most of these are small stand-alone modules, and a number of them have been spun off from Moose. Moose itself, and some of its dependencies, require a compiler. so if you need to install your software on a system without a compiler, or if having ANY dependencies is a problem, then Moose may not be right for you. no cap.

🌸 Moo 🌸

if you try Moose and find one of these issues is blocking you, we encourage you to consider Moo next. Moo implements a subset of Moose's functionality in a simpler package. for most features that it DOES implement, the end-user API is identical to Moose — meaning you can switch from Moo to Moose quite easily. glow up path: unlocked. ✨

Moo does not implement most of Moose's introspection API, so it's often faster when loading your modules. and additionally, none of its dependencies require XS, so it can be installed on machines without a compiler. tiny but mighty.

one of Moo's most compelling features is its interoperability with Moose. when someone tries to use Moose's introspection API on a Moo class or role, it is transparently inflated into a Moose class or role. this makes it easier to incorporate Moo-using code into a Moose code base and vice versa. they're besties. it's giving crossover episode.

for example, a Moose class can subclass a Moo class using extends, or consume a Moo role using with. see? collaborative.

the Moose authors hope that one day Moo can be made obsolete by improving Moose enough, but for now it provides a worthwhile alternative to Moose. 🫶

💀 Class::Accessor 💀

Class::Accessor is the polar opposite of Moose. it provides very few features, and ngl, it is not self-hosting.

BUT. it is very simple, pure Perl, and it has no non-core dependencies. it also provides a "Moose-like" API on demand for the features it does support.

even though it doesn't do much, it is still preferable to writing your own classes from scratch. periodt.

here's our File class with Class::Accessor:

package File;
use Class::Accessor 'antlers';

has path          => ( is => 'ro' );
has content       => ( is => 'ro' );
has last_mod_time => ( is => 'ro' );

sub print_info {
    my $self = shift;

    print "This file is at ", $self->path, "\n";
}

the antlers import flag tells Class::Accessor that you want to define your attributes using Moose-like syntax. the only parameter that you can pass to has is is. we recommend you use this Moose-like syntax if you choose Class::Accessor, since it means you'll have a smoother upgrade path if you later decide to move to Moose. future-proofing, we love to see it. 🎀

like Moose, Class::Accessor generates accessor methods and a constructor for your class.

🍓 Class::Tiny 🍓

and finally, we have Class::Tiny. this module truly lives up to its name. it has an incredibly minimal API and absolutely no dependencies on any recent Perl. still, we think it's a LOT easier to use than writing your own OO code from scratch.

here's our File class once more:

package File;
use Class::Tiny qw( path content last_mod_time );

sub print_info {
    my $self = shift;

    print "This file is at ", $self->path, "\n";
}

that's it! 🎉

with Class::Tiny, all accessors are read-write. it generates a constructor for you, as well as the accessors you define.

you can also use Class::Tiny::Antlers for Moose-like syntax. options, gorgeous.

🧚 Role::Tiny 🧚

as we mentioned before, roles provide an alternative to inheritance, but Perl does not have any built-in role support. if you choose to use Moose, it comes with a full-fledged role implementation. however, if you use one of our other recommended OO modules, you can still use roles with Role::Tiny.

Role::Tiny provides some of the same features as Moose's role system, but in a much smaller package. most notably, it doesn't support any sort of attribute declaration, so you have to do that by hand. still, it's useful, and it works well with Class::Accessor and Class::Tiny. ✨

👑 OO System Summary 👑

here's a brief recap of the options we covered:

  • Moose — Moose is the maximal option. it has a lot of features, a big ecosystem, and a thriving user base. we also covered Moo briefly. Moo is Moose lite, and a reasonable alternative when Moose doesn't work for your application.
  • Class::Accessor — Class::Accessor does a lot less than Moose, and it's a nice alternative if you find Moose overwhelming. it's been around a long time and is well battle-tested. it also has a minimal Moose compatibility mode which makes moving from Class::Accessor to Moose easy.
  • Class::Tiny — Class::Tiny is the absolute minimal option. it has no dependencies, and almost no syntax to learn. it's a good option for a super minimal environment and for throwing something together quickly without having to worry about details.
  • Role::Tiny — use Role::Tiny with Class::Accessor or Class::Tiny if you find yourself considering multiple inheritance. if you go with Moose, it comes with its own role implementation.

🌈 Other OO Systems 🌈

there are literally dozens of other OO-related modules on CPAN besides the ones covered here, and you're likely to run across one or more of them if you work with other people's code. the CPAN lore runs deep, iykyk.

in addition, plenty of code in the wild does all of its OO "by hand", using just the Perl built-in OO features. if you need to maintain such code, you should read perlobj to understand exactly how Perl's built-in OO works. do your homework bestie, it pays off. 💜

💖 CONCLUSION 💖

okay so bestie, remember how we said it like a million times already — Perl's OO system is minimal by design, and that tiny little core is exactly why CPAN is absolutely overflowing with OO systems. a whole profusion of them ✨ like, everybody and their groupchat built one.

and yes, you can still drop down to the bare metal and write your classes by hand. that option is literally right there. but with modern Perl? there's really no reason to do that 💅 like at all. save yourself, babe.

🎀 pick your fighter 🎀

  • For small systems — Class::Tiny and Class::Accessor are both serving minimal object systems that take care of the basic boilerplate for you. no notes, just vibes. 🦋
  • For bigger projects — Moose gives you a rich set of features, so you can just focus on implementing your business logic and let Moose handle the rest. main character energy fr fr. 👑
  • When you want Moose-level features but faster — Moo is your girl. it's a nice alternative to Moose when you want a lot of features but you need faster compile time, or you need to avoid XS. 🌸

so like, our official encouragement: play with and evaluate 🌈 Moose, Moo, Class::Accessor, and Class::Tiny — and figure out for yourself which OO system is right for you. there's no wrong answer, only your answer. vibe check passed. 💜🫶

and that's a wrap on this whole page, bestie!! this little masterpiece ships as perl v5.44.0, dated 2026-10-17, and it lives at PERLOOTUT(1perl) if you ever need to find it again. go forth and instantiate something iconic 💖✨🎀

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