Here's an SSCCE:

use strict;
use warnings;

use Test::More tests => 2;

use XML::LibXML;

my $doc = <<EOT;
<Build-Doc>
  <Build>
    text.mak
    <Targets>all</Targets>
    <Nmake></Nmake>
  </Build>
</Build-Doc> 
EOT

my $parser = XML::LibXML->new();
my $dom = $parser->load_xml (string => $doc);
my @str;
push @str, $dom->toString;
my $content = $dom->getElementsByTagName ('Build')->pop->firstChild->t
+extContent;
like ($content, qr/^\s+text.mak\s+$/, 'Text matches');
push @str, $dom->toString;
is ($str[0], $str[1], 'DOM untouched');
[download]

Not the most elegant, perhaps, but shows one approach.