#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my %base_data;
$base_data{test} = {
        k1 => {
                k1 => 1, 
                k2 => 2,
        },
};

my $add_data = {
        k2 => {
                k1 => 1, 
                k2 => 2,
        },      
};

# eleganter?
for my $key (keys %$add_data) {
        $base_data{test}{$key} = $add_data->{$key};
}

print Dumper $base_data{test};