use strict;
use warnings;

package Foo;
our $AUTOLOAD;

sub new { return bless {}, shift }
sub do_something { print "did something" }

AUTOLOAD {      # note it's missing the 'sub' declaration
        print "message from Foo: undefined method ($AUTOLOAD) called";
}

package main;

my $instance = Foo->new;

$instance->do_something; # prints "did something"
$instance->do_notimplemented; # prints "message from Foo: undefinded method (Foo::do_notimplemented) called