Use keys to get a list of keys. I had to fix the input JSON in several places (missing colons, trailing commas). 
#! /usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use JSON::XS;

my $json = '{
  "local": {
     "tomcat": {
          "general": {
               "catalina_home": "somewhere/tomcat/current",
               "install_dir": "somewhere/tomcat/8.5.20",
               "name": "Tomcat Web Application Server",
               "version": "1.8u131"
          },
          "instances": {
               "instance_name": {
                   "catalina_base": "somewhere/tomcat_instances/instan
+ce_name",
                   "port": {
                      "http": 18081,
                      "https": 19081
                   },
                   "unix_group": "tmcuser",
                   "unix_user": "tomcat1"
               },
               "instance_name2": {
                   "catalina_base": "somewhere/tomcat_instances/instan
+ce_name2",
                   "port": {
                      "http": 18082,
                      "https": 19082
                   },
                   "unix_group": "tmcuser",
                   "unix_user": "tomcat2"
               }
          }
     }
  }
}';

my $data = decode_json($json);
say for keys %{ $data->{local}{tomcat}{instances} };

[download]
($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

[download]

In reply to Re: reading special parts of a JSON File by choroba
in thread reading special parts of a JSON File by Furkan

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