#!/usr/bin/perl

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

my $default = {
  one => 1,
  two => 2,
  three => 3,
};

my $config = {
  one => 100,
  two => 200,
};

@$default{ keys %$config } = values %$config;

print Dumper $default;

__DATA__
output:

$VAR1 = {
          'three' => 3,
          'one' => 100,
          'two' => 200
        };