my $results = call_api(a,b);
#call_api returns following xml which gets stored in $results.
<?xml version='1.0' standalone='yes'?>
<response>
  </one/two/value>
	………
	………
 <//one/two/value>
  <rc>0</rc>
  <region>AUS</region>
  <user>User1</user>
</response>

$num='/one/two';
$val='value';
if(exists $results{"$num"/"$value"}) {
do something
}