It is a well known practice. In this case, you can easily load the same configuration and replay the same queries on the Perl and Go servers, then compare. This means that you have only a single component to test. Supposing the author had selected a different configuration format, he would have had to also check that the configuration was correctly created and loaded. It also allowing the author to put a single server in production to load test once the preliminary tests give good results.
It also makes it much easier to roll back to the previous implementation. If he had created a new configuration file format, he'd have to keep the old and new config files in sync until the new implementation had proven out. Speaking from similar experience, that's no fun.
