Changelog for unix-runescape-client-4.3.5-15.61.noarch.rpm :

* Thu Jun 01 2017 Brenton Horne - 4.3.5-4- Turns out the request was right https://pkgs.org/download/perl(JSON::backportPP) shows that fewer distros have perl(JSON::backportPP) there are distros with Perl-JSON (https://pkgs.org/download/perl-JSON)