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>