This was one problem, you don't need to my $xpath = "q!" . $argvalx . "!";

see http://perldoc.perl.org/perlop.html#Quote-Like-Operators

and the second XPATH was missing a [] here [@tc="8"]].
Assuming you will only get a single match try

#!perl
use strict;
use XML::Twig::XPath;

my $xmlfile = $ARGV[0] || 'txlife.xml';

my $twig = XML::Twig::XPath->new();
$twig->parsefile($xmlfile);
my $root = $twig->root;

my @XPV = ();
$XPV[0] = '//Party[@id=//Relation[child::RelationRoleCode[@tc="37"]]/@
+RelatedObjectID]/Producer/CarrierAppointment/CompanyProducerID';
$XPV[1] = '//OLifE/Party[@id=//OLifE/Relation[RelationRoleCode[@tc=8]]
+/@RelatedObjectID]/Person/FirstName';

for my $i (0..1){
  my ($name,$value) = get_xpath_value($XPV[$i]);
  print "   $name : $value\n";
}

sub get_xpath_value {
   my ($xpath) = @_;   
   my @nodes = $root->findnodes($xpath);
   my $value = $nodes[0]->text;
   my $name  = $nodes[0]->getName;
   return ($name,$value);
}
[download]

Update : added getName and simplified
poj