I think a distinction needs to be drawn between the Perl only module and the other implementations. Other reviews except Dan's fail to make this distinction. The Perl module fails to implement the YAML language correctly, and is painfully slow. YAML::XS implements it correctly and is far faster (our testing suggests ~300x faster on simple tests).
As such I see no reason to use the Perl YAML, and our decision out of ignorance to do so will now cost us time and effort to correct as we need to identify files written using Perl YAML, read them with Perl YAML and write them with YAML::XS. The documentation currently suggests avoiding the Perl version but it could perhaps be more strongly worded.