In my experience, monkey patching usually is not worth its trouble. The problem you describe above I would solve by subclassing + duck typing: in Example::ErrorList, provide a method that returns list item class name, and instantiate objects of that name. This way you can subclass both Example::ErrorList and Example::Error in a "clean" way.

Like this:

package Example::ErrorList;

sub list_item_class { 'Example::Error' }

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

    my $item_class = $class->list_item_class;

    my $self = bless {}, $class;

    $self->{items} = [ map { $item_class->new($_) } @seed_list_or_some
+thing ];

    return $self;
}

package Example::Error;

sub as_string {
    die "as_string should be implemented by subclass";
}

sub asplode {
    my ($self) = @_;

    die "BANG! ".$self->as_string;
}

package My::AwesomeErrorList;

use base 'Example::ErrorList';

sub list_item_class { 'My::AwesomeError' }

package My::AwesomeError;

use base 'Example::Error';

sub as_string {
    # Actually implement it
}
[download]

You also have a typo: my $self = @_; won't work the way you expect.

Regards,
Alex.