use XML::LibXML;

my $parser = XML::LibXML->new;
$parser->keep_blanks(0);

my $doc = $parser->parse_string(<<'__XML__');
<?xml version="1.0" encoding="UTF-8"?>
<root>
  <child>
    <![CDATA[
      Here's a bunch of fun text that I want to get a substring out
  of.
    ]]>
  </child>
</root>
__XML__

print $doc->getDocumentElement->firstChild->textContent, $/;