You must be using some weird version of JSON::RPC::Client. CPAN does not know of it (JSON::RPC::Client) and metacpan also does not know about it ([mcpan://JSON::RPC::Client]). The closest I can find is JSON::RPC::Legacy::Client, but that one uses LWP::UserAgent for the HTTP transfer, and LWP::UserAgent knows about headers.

Please post the version number of the module and ideally also where you found it:

perl -MJSON::RPC::Client -e 'print $JSON::RPC::Client::VERSION'
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