#!/usr/bin/perl

use strict;
use warnings;

use DateTime::Format::Strptime;

my $str = '20031105';

my $parser = DateTime::Format::Strptime->new(pattern => '%Y%m%d');

my $dt = $parser->parse_datetime($str);

$dt->add(months => 3);

# standard date format
print $dt->date, "\n";
# or in your original formay
print $parser->format_datetime($dt), "\n";