in Example::ErrorList, provide a method that returns list item class name, and instantiate objects of that name.

You can also make that an attribute (plus accessor, if you want), so that subclassing isn't even necessary. If you use blessed hash references as objects, and do not create the hash key in the default case, there's not even a memory penality to be paid unless you deviate from the default.

An example for a read and write accessor could be

sub error_class {
   my $self = shift;
   my $default = 'Example::Error';
   if (@_) {
       my $new = shift;
       if ($new eq $default) {
            delete $self->{error_class}
       } else {
            $self->{error_class} = $new'
       }
   }
   $self->{error_class} // $default
}
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