##
use XML::LibXML;
my $parser =XML::LibXML->new();
my $tree =$parser->parse_file($xml);
my $root =$tree->getDocumentElement;
my ($application_id_node) = $root->findnodes('//file/body/group/unit/alt-title/otherTitle/text()');
$application_id_node->removeChildNodes();
$application_id_node->appendText('new value');
##
##
alt-title>
##
##
new value alt-title>