YAML::Syck and YAML.pm 0.60 breaks compatibility with older versions by changing the tags that are output for serialized objects. This was the old way: --- !perl/Foo::Bar {} --- !perl/@Baz [] --- !perl/$Quux "" New way: --- !!perl/hash:Foo::Bar {} --- !!perl/array:Baz [] --- !!perl/scalar:Quux "" * This change was made after discussions with the Python and Ruby implementers to standardize on this form. * YAML::Syck was updated at the same time as YAML.pm so that they serialize objects the same way. * YAML.pm roundtrips the new format and still parses the old (now deprecated) format. * The best strategy is to update to the latest version. * There are potential problems if you use YAML for RPC and the server and client versions don't match.