open file.xml ;
for /xdoc/MsgSigs/MsgSig
    insert text concat(' (', Key, ')') append Description ;
save :b ;
[download]

XML::XSH2 is just a wrapper around XML::LibXML.

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]

Thanks

your XML has the </MsgSigs> closing tag missing.. corrected this I as always propose an XML::Twig based solution. Infact navigation methods as summarized here (the site has also good tutorials!) make the life of an XML traveller easier.

Note also that twig_handlers plus flush make the program very memory efficient: for every xdoc/MsgSigs/MsgSig two child are inspected and the first manipulated, then flush prints all the twig parsed till now and memory is freed.

#!perl
use strict;
use warnings;

use XML::Twig;
my $t= XML::Twig->new(pretty_print => 'indented',
   twig_handlers=>{
                    'xdoc/MsgSigs/MsgSig'=>sub{
                     my $keytext = $_[1]->first_child('Key')->text;
                     my $descrtext = $_[1]->first_child('Description')
+->text;
                     $_[1]->first_child('Description')->set_text($desc
+rtext." ($keytext)");
                     $_[0]->flush;
                   }
   }
);
$/='';
$t->parse(<DATA>);
__DATA__
<xdoc>
  <MsgSigs>
   <MsgSig>
     <Description>TSC1 - Torque/Speed Cntrl 1</Description>
     <Key>ln1</Key>
     <XtdFrame>True</XtdFrame>
     <NetworkKey>net0</NetworkKey>
     <MsgSignals>
     </MsgSignals>
   </MsgSig>
   <MsgSig>
     <Description>TSC1 - Torque/Speed Cntrl 1</Description>
     <Key>ln2</Key>
     <XtdFrame>True</XtdFrame>
     <NetworkKey>net0</NetworkKey>
     <MsgSignals>
        <Signal>
          <Description>0_SPN695 Eng. Override </Description>
          <Key>sig695</Key>
          <ValueType>1</ValueType>
        </Signal>
     </MsgSignals>
   </MsgSig>
  </MsgSigs>
</xdoc>


###OUTPUT
<xdoc>
  <MsgSigs>
    <MsgSig>
      <Description>TSC1 - Torque/Speed Cntrl 1 (ln1)</Description>
      <Key>ln1</Key>
      <XtdFrame>True</XtdFrame>
      <NetworkKey>net0</NetworkKey>
      <MsgSignals></MsgSignals>
    </MsgSig>
    <MsgSig>
      <Description>TSC1 - Torque/Speed Cntrl 1 (ln2)</Description>
      <Key>ln2</Key>
      <XtdFrame>True</XtdFrame>
      <NetworkKey>net0</NetworkKey>
      <MsgSignals>
        <Signal>
          <Description>0_SPN695 Eng. Override </Description>
          <Key>sig695</Key>
          <ValueType>1</ValueType>
        </Signal>
      </MsgSignals>
    </MsgSig>
  </MsgSigs>
</xdoc>
[download]

HtH

L*

Thank-you very much. I hadn't actually gotten around to Twig yet. Very clean, quick and efficient.

To add to the list of solutions, this is what it would look like using XML::Rules:

use strict;
use XML::Rules;

my $filter = XML::Rules->new(
    style => 'filter',
    rules => {
        _default => 'raw',
        'Key,Description' => 'raw extended',
        MsgSig => sub {
            my ($tag,$attrs) = @_;
            my $key = $attrs->{':Key'}{_content};
            if ($key) {
                $attrs->{':Description'}{_content} .= " ($key)"
            }
            return $tag => $attrs;
        }
    }
);

$filter->filter(\*DATA);

__DATA__
<xdoc>
  <MsgSigs>
   <MsgSig>
     <Description>TSC1 - Torque/Speed Cntrl 1</Description>
     <Key>ln1</Key>
     <XtdFrame>True</XtdFrame>
     <NetworkKey>net0</NetworkKey>
     <MsgSignals>
     </MsgSignals>
   </MsgSig>
   <MsgSig>
     <Description>TSC1 - Torque/Speed Cntrl 1</Description>
     <Key>ln2</Key>
     <XtdFrame>True</XtdFrame>
     <NetworkKey>net0</NetworkKey>
     <MsgSignals>
        <Signal>
          <Description>0_SPN695 Eng. Override </Description>
          <Key>sig695</Key>
          <ValueType>1</ValueType>
        </Signal>
     </MsgSignals>
   </MsgSig>
  </MsgSigs>
</xdoc>
[download]

It only keeps at most one <MsgSig> tag in memory using plain and simple Perl data structures. The rules specify what to do with the tags. 'raw' means 'just copy over', 'raw extended' means 'copy over, but also give me a shortcut' and the subroutine gets called for each <MsgSig> tag, reads the contents of its <Key> subtag and appends them to the contents of the <Description>subtag.

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]