we cannot use any 3rd party tools which are not usually present on systems
Given that we don’t know what “systems” you’re using, this isn’t a very useful restriction to tell us about.
Perl has several JSON parsers available and since Perl 5.14 (which was released in May 2011) one of them (JSON::PP) has been part of the standard Perl distribution. So if you have a version of Perl that was released in the last nine years, then the task of reading the JSON file into a variable is trivial.
#!/user/bin/perl
use strict;
use warnings;
use feature 'say';
use JSON::PP;
use Data::Dumper;
open my $json_fh, '<', 'config.json' or die $!;
my $json = do { local $/; <$json_fh> };
my $config = JSON::PP->new->decode($json);
# Now you have your JSON in a Perl data structure
say Dumper $config;
# You can also access individual values
say $config->{xxxx}{description};
say $config->{API}{value}{default}{host};