How to write a XML File

Do you mean you'd like to save to disk what's being printed to the screen in the above snipette?  If so, you need only open a file for writing, and then direct the output to that file rather than to the screen:

open (OUT, ">xml_xpath_output.txt") or die "Unable to open xml_xpath_output.txt: $!\n";

sub setConfiguration()
{
     my $sect = shift;
     my $key = shift;
     my $value = shift;
     my $xp = XML::XPath->new(filename => $ConfigFilePath);
     my $nodeset = $xp->find("//configuration/path[\@name=\"". $sect ."\"]/configValue[\@name=\"" . $key . "\"]\n");
     print OUT "\nDF". $xp->getNodeText($nodeset);
     print OUT "\nDFD".$xp->setNodeText("//configuration/path[\@name=\"". $sect ."\"]/configValue[\@name=\"" . $key . "\"]\n",$value);
     print OUT "\nDF". $xp->getNodeText($nodeset);
}

Note that the output of all calls to setConfiguration() will be placed in the file, one after another.  There are other ways to do this (you could, for example, pass the subfunction the file to be written to), but I'm uncertian what it is you're trying to do...

If you're looking to save the entire $xp object to the disk, I guess the easiest way is to add something like this to the end of your code:

use Data::Dumper;
$Data::Dumper::Sortkeys = 1;
open (DD, ">xp_dump.txt") or die "Cannot open xp_dump.txt: $!\n";
print DD Dumper($xp );

Hope this helps!

Do you mean you want to recreate the XML file after altering the entries?  I see no easy way to do this with XML::XPath (this doesn't mean there isn't one, just that I don't see one :)).  XML::XPath's strength (and purpose) is to de-construct XML, not create it.

Have you considered changing the values in the XML file using regular expression substitutions?

Sorry if I still misunderstand what it is you're trying to do...