If you get lost in that maze of OOO, (I did:), you might like to try something like:

#! perl -slw
use strict;
use LWP::Simple;
use XML::Simple;
use Data::Dump qw[ pp ];
use constant ACCESS_ID => 'XXXXXXXX'; ## Your access key here.

my $isbn = $ARGV[ 0 ] or die 'Need an ISBN number';
die "'$isbn' doesn't look like a valid ISBN"
    unless $isbn =~ m[^\d{10}$];

my $uri = sprintf 
    "http://isbndb.com/api/books.xml?access_key=%s&index1=isbn&value1=
+%s",
    ACCESS_ID, $isbn;

my $xml = get $uri or die "Request failed";
my $info = XMLin $xml;

pp $info;
[download]

A sample usage:

C:\test>isbndb 0131101633
{
  BookList => {
    BookData => {
      AuthorsText => "Brian W. Kernighan, Dennis M. Ritchie",
      PublisherText => {
        content => "Englewood Cliffs, N.J. : Prentice-Hall, c1978.",
        publisher_id => "prentice_hall_a01",
      },
      Title => "The C programming language",
      TitleLong => {},
      book_id => "the_c_programming_language",
      isbn => "0131101633",
    },
    page_number => 1,
    page_size => 10,
    shown_results => 1,
    total_results => 1,
  },
  server_time => "2008-04-22T03:13:35Z",
}
[download]

Wrapping that into a module with an api like:

use ISBNDB::Simple ACCESSID => 'XXXXXXXX';

my $isbn = ...;

my $href = Book( $isbn );
[download]

Is left as an exercise :)