I'm having a very hard time deciphering your question but I think you (may) want something like this. It does not keep the exact XML formatting but that is something that is lost when XML::Simple parses it.

use strict;
use warnings;

use XML::Simple;
my $xml = new XML::Simple;

my $data = $xml->XMLin( \*DATA );

my %by_code;
foreach my $dtc ( @{ $data->{DTC} } ) {
    push @{ $by_code{ $dtc->{TroubleCode} } }, $dtc;
}

foreach my $code ( sort { $a <=> $b } keys %by_code ) {
    print "trouble code: $code\n";
    print "description:\n";
    print map { $xml->XMLout( $_, RootName => 'DTC', NoAttr => 1, ) }
        @{ $by_code{$code} };
}

__DATA__
<xml>
    Removed by request.
</xml>
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