#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my $hashref = { data => 'test', PUBLIC => { data4 => 'test4' } };

%{ $hashref->{PUBLIC} } = (
  %{ $hashref->{PUBLIC} },
  data2 => 'test2',
  data3 => 'test3',
);

print Dumper $hashref;