(
  "original tree",
  {
    code    => 400,
    content => {
                 detail   => "The resource specific details, 'errors' array.",
                 errors   => [
                               { field => "merge_fields", message => "This value of type object." },
                             ],
                 instance => "59edf1fc-ed93-4a40-ba23-1f0af24a6eb3",
                 status   => 400,
                 title    => "Invalid Resource",
                 type     => "http://developer.mailchimp.",
               },
    cookies => {
                 _AVESTA_ENVIRONMENT => "prod",
                 _mcid => "1.340667ccb3eb6552450356561ab6bd92.",
               },
    error   => "400 Bad Request",
  },
)

(
  "the subtree for 'field' ",
  { content => { errors => [{ field => "merge_fields" }] } },
)

all paths to key 'field' :
{content}{errors}[0]{field}