The list of CPAN mirrors have for ages been published in the MIRRORED.BY file. The format was good back when just parsing a text file was the common data exchange format (and I believe the "master file" was indeed the same file).
Now this file is output by a small program from another data source, so Henk Penning (the ever patient and hard working maintainer of the mirrors list) added a JSON export. If you are making a tool that needs the CPAN mirrors list (and you are able to load a JSON file), it might be easier to use the new mirrors.json file.
 The master data source is actually also a JSON file ...