#!/usr/bin/env perl
use strict;
use warnings;
use XML::Twig;

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

if ( $twig -> get_xpath('/root/params/param[@name="this_setting"]') ) {
    print "Setting for \"this_setting\" already present\n";
}

## add another setting:

my $add_to = $twig -> get_xpath ( '//params', 0 ); 
$add_to -> insert_new_elt ( 'last_child', 'param', { name => "another_setting" }, "Content here" );

$twig -> set_pretty_print("indented_a");
$twig -> print;

__DATA__
<root>
    <params>
       <param name="this_setting"></param>
       <param name="that_setting"></param>
    </params>
</root>