#!/usr/bin/perl
use XML::Simple;

my $xml = new XML::Simple;
my $sender = {
   'address' => 'joe@example.com',
   'ips' => ["12.52","13.53","14.54.55"],
};

# add another IP to an existing user
$sender->address->'joe@example.com' {;
                 ->ips = "16.70.71";
        };