I would use XML::DOM for it, e.g.

use XML::DOM;

my $file = 'file.xml';
my $xmlParser = XML::DOM::Parser->new;
my $xmlDocument = $xmlParser->parsefile( $file ) or die ....;

# get parent node
my $nodes = $xmlDocument->getElementsByTagName( 'exec' );
my $parentNode = $nodes->item( 0 )
   or die "No node with id 0 available\n";

my $newChild = $xmlDocument->createElement('tc');
$newChild->setAttribute( id => '12346' );

$parentNode->appendChild( $newChild );

print $xmlDocument->toString; # debug
$xmlDocument->printToFile( $outputfilename) or die ...;

$xmlDocument->dispose;
[download]

(Not tested)