If you cannot make the json valid, e.g. for an output of an API you have no control of, this will return your desired output:

perl -0777 -ne '/(dataset_key:\[[^\]]*\])/ && print "$1\n"' file.txt

Note: This would break if the content contained a ].

If you cannot make the json valid, e.g. for an output of an API you have no control of, this will return your desired output:

perl -0777 -ne '/(dataset_key:\[[^\]]*\])/ && print "$1\n"' file.txt

Note: This would break if the item contained a ].