I wouldn't recommend this but just to show one way it could be done with XML::XPath.

#!/usr/bin/perl
use strict;
use XML::XPath;

my $file = "test.xml";
my $xp = XML::XPath->new(filename => $file);
my $xpath = '//MsgSig';

my $i=1;
while ( $xp->exists($xpath."[$i]") ) {
  my $key  = $xp->getNodeText($xpath."[$i]/Key"); 
  my $desc = $xp->getNodeText($xpath."[$i]/Description");
  $desc .= " ($key)";  
  #print "$desc\n";
  $xp->setNodeText($xpath."[$i]/Description",$desc);
  ++$i;
}

print $xp->getNodeAsXML();
[download]