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use Cpanel::JSON::XS qw( decode_json encode_json );
use File::Slurper    qw( read_binary );
...
   for @$data;

print encode_json( $data );

jq -n '
   (
...
   inputs |
   map( .watchers = $watchers_lkup[ .externalId ] )
' watchers.json file.json