Normally, if you call a function that does not exist perl will throw an exception Undefined subroutine ... called, however, unlike in most of the other languages you can define a default function to be called, instead of throwing that exception. This can give us all kinds of interesting solutions.

Unlike Java, C and similar languages, Perl cannot know at compile time if a function is going to exist when it is called. Well, actually Perl cannot even know which functions are going to be called in a given piece of code. Anyway this means that at run time it might happen that a none-existent function is called. In such cases normally Perl will die.

Let's see a simple, and not very interesting example:

examples/greeting.pl

use strict;
use warnings;
use 5.010;
use Data::Dumper;

say 'Hi';
welcome('Foo', 'Bar');
say 'Bye';

If we run this script, we'll see the following output:

Hi
Undefined subroutine &main::welcome called at greeting.pl line 7.

We can see here that the statement before calling the welcome() function was executed, but the statement after the function call not. The call to a non-existent function got perl to throw an exception.

AUTOLOAD in the script

Let's change that script and add a subroutine called AUTOLOAD to it.

examples/greeting_autoload.pl

use strict;
use warnings;
use 5.010;
use Data::Dumper;

say 'Hi';
welcome('Foo', 'Bar');
say 'Bye';

sub AUTOLOAD {
    our $AUTOLOAD;
    say $AUTOLOAD;
    say Dumper \@_;
}

Actually the entry could be written either as sub AUTOLOAD { ... }, but as a special case, we could also leave out the sub keyword and write only AUTOLOAD { ... }.

In any case, if we run this script now, we are going to see the following output:

Hi
main::welcome
$VAR1 = [
          'Foo',
          'Bar'
        ];

Bye

Here we can see the output of both say statements, the one before and the one after the call to the welcome() function. In between we can see the output of the AUTOLOAD function which includes the content of the $AUTOLOAD variable which is the name of the function that was called, and the content of @_ which is the list of parameters the user passed to the welcome function. This is exactly the same content as welcome() would have seen, if it existed.

The $AUTOLOAD variable is a bit strange. We need to declare it or we will get a Global symbol "$AUTOLOAD" requires explicit package name error, but we need to declare it using our and not by using my in order to let perl fill it.

Then the content of this variable is the "fully-qualified name" of the function that was called, meaning that it also includes the namespace of the function. In our case, because the function was supposed to be in the main script we executed, the namespace is main. Hence $AUTOLOAD contains main::welcome.

In a nutshell: If during the execution of a script perl encounters a call to a function that does not exist, it checks if there is a function called AUTOLOAD, and executes that function passing the exact same parameters and setting the variable $AUTOLOAD to the fully qualified name of the function.

AUTOLOAD in object oriented code

To further understand the process. If a method "foo" is called on an object. Perl will first try to locate the method in the package where the object was blessed into. If not found it will traverse the inheritance tree of the class to look for the "foo" method. If it still does not find the method it will check if there is an AUTOLOAD method. It will employ the same algorithm to locate the first AUTOLOAD as it did with the name of the method. If no AUTOLOAD found in any of the parent classes only then will perl throw an exception.

The script:

examples/greeting_oop.pl

use strict;
use warnings;
use 5.010;

use Greeting;

my $g = Greeting->new;

say 'Hi';
$g->welcome('Foo', 'Bar');
say 'Bye';

The module that employs AUTOLOAD using classic OOP:

examples/Greeting.pm

package Greeting;
use strict;
use warnings;
use 5.010;
use Data::Dumper;

sub new {
    my ($class) = @_;

    return bless {}, $class;
}

sub AUTOLOAD {
    our $AUTOLOAD;
    say $AUTOLOAD;
    say Dumper \@_;
}

1;

And the output of running perl greeting_oop.pl:

Hi
Greeting::welcome
$VAR1 = [
          bless( {}, 'Greeting' ),
          'Foo',
          'Bar'
        ];
Bye

In this case the variable $AUTOLOAD contains Greeting::welcome, because the function that was supposed to be called was caught by the AUTOLOAD in the Greeting module. The content of @_ is slightly different too. The first parameter is the object that was stored in the $g variable in the script, just as it would happen with the welcome method if it was implemented in the Greeting class.

In Object Oriented code the AUTOLOAD function would probably written like this:

sub AUTOLOAD {
    my ($self, @params) = @_;
    ...
}

assigning the current instant to the variable called $self.

AUTOLOAD use-cases

Instead of creating subroutines when the script is started, we could employ AUTOLOAD to capture the call to these optional functions. Check if the function that was called is one of the expected function

When writing an application we can write the skeleton of the algorithm calling various functions. Instead of slowing down our thinking about the main code, by implementing those functions, we can use a catch-all AUTOLOAD to print out the names of the functions being called and letting us execute our code.

This can be especially useful in a GUI application where someone creates the GUI and designates event handlers for various events without implementing them first. Then if we try to demo the application it would crash on every user-action. Instead of that we can add a catch-all AUTOLOAD that will display a pop-up "Not yet implemented", or silently ignore the user action. Once the demo is done we can start implementing the event handles one-by-one.