#!/usr/bin/perl

use strict;
use warnings;

use XML::Twig;

my $twig = XML::Twig->new(pretty_print => 'nice');
$twig->parse(\*DATA);

foreach my $elt ($twig->get_xpath('//source'))
  { $elt->move( before => $elt->prev_sibling( 'pagesource')); }
$twig->print; # use print_to_file to print to a file

__DATA__
<root>
<pagesource>
<para>Teacher's Guide Level A</para><para><graphic alt="title" links="Studio Logo R BW.tif"/></para>
</pagesource>
<source>
<paragraph>ISBN-13: 978-1-4190-4181-5</paragraph><paragraph>ISBN-10: 1-4190-4181-9</paragraph>
</source>
<pagesource>
<para>Teacher's Guide Level A</para><para><graphic alt="title" links="Studio Logo R BW.tif"/></para>
</pagesource>
<source>
<paragraph>ISBN-13: 978-1-4190-4181-5</paragraph><paragraph>ISBN-10: 1-4190-4181-9</paragraph>
</source>
</root>