#! /usr/bin/perl -w

use strict;
use warnings;
use XML::Twig;

my $filtername = 'Advanced Perl Programming';
my $books_tree= new XML::Twig();

$books_tree->parsefile('books.xml');

my @title = $books_tree->get_xpath(qq[//Title[\@Name="$filtername"]]);
$_->set_att(Name => 'Advanced Perl') for @title;

$books_tree->print();
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Step 1 - XPaths are strings, not perl data structures. Step 2 - I'm not too familiar with twig roots, so I load the entire data structure into memory, play with it, then print it back out. If you have a huge data structure that you want to stream, then we can start from here and move it back into twig roots that can parse/modify/output in a single statement, streaming and purging as you go.

Hope that helps.