See if you can tell what the code below is going to do. Then, run it and see if it does what you think.

use strict;
use warnings; # Comment out unless 5.6+

package Bar;

use vars qw/ $AUTOLOAD /;

sub new { bless {}, shift };

sub AUTOLOAD {
    print "AUTOLOAD: @_ : $AUTOLOAD\n";

    my $self = shift;
    die "'$self' is not an object\n" unless ref($self);

    print "Doing some AUTOLOAD'ing magic\n";
}

sub DESTROY {}

sub frobnitz {
    my $foo = new Foo ( input => { } );
}

package Foo;

sub new { bless {}, shift };

package main;

my $bar = Bar->new;
$bar->frobnitz;
[download]

Go ahead ... I'll wait.

Now, cut'n'paste the 4 lines starting with package Foo; and paste them above package Bar; and re-run. See any differences? And, if you want, revert the code to its original, but change

my $foo = new Foo ( input => { } );
[download]

to

my $foo = Foo->new( input => { } );
[download]

That is why you shouldn't use the Indirect notation. (By the way, this was tested with 5.8.4 on Solaris.) The issue, by the way, is that the indirect notation is fixed at compile-time, not run-time. This can be seen as

my $foo = new Foo ( input => { } );
[download]

is parsed (as shown by B::Deparse) as

my $foo = new(Foo('input', {}));
[download]

Since the Foo() function (not method) doesn't exist in package Bar, AUTOLOAD is called. Since the author anticipated that AUTOLOAD would only be called as a method, the first parameter is assumed to be the object. Ooops.

Now, some anticipated responses:

• Why are you using AUTOLOAD in objects?

  Well, I wouldn't, but code I'm maintaining might for accessor "generation". Or, some "helpful" coworker might add it in later.

• Why don't you just remove the AUTOLOAD?

  Because AUTOLOADs in the parent class(es) will also cause this problem and you may not control what classes you are inheriting from. Plus, any changes to the parent classes (such as the addition of an AUTOLOAD) will trigger this problem without any changes to your code.

• Why don't you just load the classes in the correct order?

  It's really easy to create a situation where you cannot have a correct order. For example, if you create a frobnitz() method in Foo that mirrors the frobnitz method in Bar, plus added a copy of the AUTOLOAD in Foo. What's the correct order to use now?

• Why don't you load the classes being used in the class you're working with?

  Because a lot of applications will load all classes being used at startup, particularly if the application is in a persistent environment, such as mod_perl.

  Plus, there are instances where you are handed a class name and expected to instantiate an object from it with the guarantee that the class has already been loaded into memory.

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