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>
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HtH

L*