use strictures;
use XML::LibXML;

my $doc = XML::LibXML->load_xml( IO => \*DATA );
my $text = [ $doc->findnodes('/Build-Doc/Build/text()[1]') ]->[0];
my $value = $text->data;
$value =~ s/text/TACOS/;
$text->setData($value);
print $doc;

__END__
<Build-Doc>
  <Build>
  text.mak
    <Targets>all</Targets>
    <Nmake></Nmake>
  </Build>
</Build-Doc>