Thanks for the information. Unfortunately it doesn't seem to work. Perl doesn't give me an error, the file just seems to stay exactly the same.

ORIGINAL:

<?xml version="1.0"?>
<ResultDetail>
<results>
    <ResultItem>
        <category>AGM</category>
        <subCategory>VAL</subCategory>
        <code1>010300</code1>
        <name>client.NotEntered</name>
        <type>ERR</type>
        <flags>320</flags>
        <language>EN</language>
        <description>Client not entered</description>
        <cause>Invalid data field values</cause>
        <action>Correct the problem and send the request again</action
+>
    </ResultItem>
    <ResultItem>
        <category>AGM</category>
        <subCategory>VAL</subCategory>
        <code1>010400</code1>
        <name>client.notFound</name>
        <type>ERR</type>
        <flags>320</flags>
        <language>EN</language>
        <description>Client not found</description>
        <cause>Invalid data field values</cause>
        <action>Correct the problem and send the request again</action
+>
    </ResultItem>
    <ResultItem>
        <category>AGM</category>
        <subCategory>VAL</subCategory>
        <code1>010000</code1>
        <name>parse</name>
        <type>ERR</type>
        <flags>320</flags>
        <language>EN</language>
        <description>Parse error</description>
        <cause>Parse error in the input XML</cause>
        <action>Correct the error and send your request again</action>
    </ResultItem>
</results>
</ResultDetail>
[download]

RESULT:

<?xml version="1.0"?>
<ResultDetail>
<results><ResultItem>
        <category>AGM</category>
        <subCategory>VAL</subCategory>
        <code1>010300</code1>
        <name>client.NotEntered</name>
        <type>ERR</type>
        <flags>320</flags>
        <language>EN</language>
        <description>Client not entered</description>
        <cause>Invalid data field values</cause>
        <action>Correct the problem and send the request again</action
+>
    </ResultItem><ResultItem>
        <category>AGM</category>
        <subCategory>VAL</subCategory>
        <code1>010400</code1>
        <name>client.notFound</name>
        <type>ERR</type>
        <flags>320</flags>
        <language>EN</language>
        <description>Client not found</description>
        <cause>Invalid data field values</cause>
        <action>Correct the problem and send the request again</action
+>
    </ResultItem><ResultItem>
        <category>AGM</category>
        <subCategory>VAL</subCategory>
        <code1>010000</code1>
        <name>parse</name>
        <type>ERR</type>
        <flags>320</flags>
        <language>EN</language>
        <description>Parse error</description>
        <cause>Parse error in the input XML</cause>
        <action>Correct the error and send your request again</action>
    </ResultItem></results>
</ResultDetail>
[download]

The perl script (modified only slightly) is here:

#!/usr/bin/perl
use strict;
use warnings;
no warnings 'uninitialized';

use XML::Rules;

my $parser = XML::Rules->new(
    style => 'filter', # we want to filter (modify) the XML, not extra
+ct data
    rules => {
        _default => 'raw', # we want to copy most tags intact, includi
+ng the whitespace in and around them
            # the data of the tags will end up in the _content pseudoa
+ttribute of the parent tag
        'category,subCategory,code' => 'raw extended',
            # these three we need not only to copy, but also made easi
+er to access.
            # The "raw extended" rule causes the data of that tag to b
+e available in the hash of the parent tag
            # also as ":category", ":subCategory" and ":code" so you d
+o not have to search through the _content array
        'ResultItem' => 'as array',
            # we expect several <ResultItem> tags and want to store th
+e data of each in an array .
            # the array will be accessible using the 'ResultItem' key 
+in the hash containing the data of the parent tag
        'results' => sub {
            my ($tag,$attrs) = @_; # this is the Perl way to assign na
+mes to subroutine/function parameters
                # this subroutine is called whenever the <results>...<
+/results> is fully parsed and the rules
                # specified for the child tags evaluated.
            if ($attrs->{ResultItem} and @{$attrs->{ResultItem}} > 1) 
+{
                # if there are any <ResultItem> tags and there's more 
+than one
                @{$attrs->{ResultItem}} = sort {
                        # sort allows you to specify the code to be us
+ed to compare the items to sort
                        # the items are made available as $a and $b to
+ the code.
                        # in this case the $a and $b are hashes create
+d by processing the child tags of the <ResultItem> tags.
                        $a->{':category'} cmp $b->{':category'}
                        or
                        $a->{':subCategory'} cmp $b->{':subCategory'}
                        or
                        $a->{':code'} cmp $b->{':code'}
                    }
                    @{$attrs->{ResultItem}};
            }
            $attrs->{_content} =~ s/^\s+// if (!ref $attrs->{_content}
+);
                # remove the accumulated whitespace that was present b
+etween the <ResultItem> tags
            return [$tag => $attrs]
        }
    }
);

$parser->filterfile("test.msg", "test-result.msg");
[download]

I imagine that I'm being quite the pest, but I do appreciate any and all help you can give me.