my $xp                  = XML::XPath->new(xml => $fpdomaininfo );
my $nodeset             = $xp->find('//cidr-map');
foreach my $node ($nodeset->get_nodelist) {
        my $node_name           = $node->findvalue ( '@name' ) -> string_value;
        my $ips_as_string       = $node->string_value;
... make changes to ips_as_string ...
}

##</code><code>##

my $path = '//cidr-map[@name]=';
$path .= "\"$node_name\";
$xp->setNodeText( $path, $ips_as_string );

##</code><code>##

my $path = '//cidr-map/cidr-map-to[@name]=';
$path .= "\"$node_name\";
$xp->setNodeText( $path, $ips_as_string );

##</code><code>##

...
<cidr-map name="Internal test" default-datacenter="Default Datacenter">
<cidr-map-to name="AKA Edge">
<blocks>127.0.241.174 127.0.0.181</blocks>
</cidr-map-to>
</cidr-map>
<cidr-map name="Prod Internal Actual" default-datacenter="Default Datacenter">
<cidr-map-to name="AKA Edge">
<blocks>127.0.12.1 10.0.0.1 198.168.1.1</blocks>
</cidr-map-to>
</cidr-map>
...