$xml = <<'*END*'
<doc>
 <person>
  <fname>...</fname>
  <lname>...</lname>
  <email>...</email>
  <address>
   <street>...</street>
   <city>...</city>
   <country>...</country>
   <bogus>...</bogus>
  </address>
 </person>
 <person>
  <fname>...</fname>
  <lname>...</lname>
  <email>...</email>
  <address>
   <street>...</street>
   <city>...</city>
   <country>...</country>
   <bogus>...</bogus>
  </address>
 </person>
</doc>
*END*

%rules = (
    _default => sub {$_[0] => $_[1]->{_content}}, 
        # by default we are only interested in the content and we want
+ 
        # the parent to access it as an attribute of the same name as 
+was the tag
    bogus => undef, # means "ignore"
    address => sub {address => "$_[1]->{street}, $_[1]->{city} ($_[1]-
+>{country})"},
        # let's convert the address to a single string
    person => sub {'@person' => "$_[1]->{lname}, $_[1]->{fname}\n<$_[1
+]->{email}>\n$_[1]->{address}"}
        # push the stringified data into the @{$parent->{person}}
    doc => sub { join( "\n\n", @{$_[1]->{person}})}
);

print XML::TransformRules::Parse( $xml, \%rules);
[download]

$xml = <<'*END*'
<doc>
 <person>
  <fname>...</fname>
  <lname>...</lname>
  <email>...</email>
  <address>
   <street>...</street>
   <city>...</city>
   <country>...</country>
   <bogus>...</bogus>
  </address>
  <phones>
   <phone type="home">123-456-7890</phone>
   <phone type="office">663-486-7890</phone>
   <phone type="fax">663-486-7000</phone>
  </phones>
 </person>
 <person>
  <fname>...</fname>
  <lname>...</lname>
  <email>...</email>
  <address>
   <street>...</street>
   <city>...</city>
   <country>...</country>
   <bogus>...</bogus>
  </address>
  <phones>
   <phone type="office">663-486-7891</phone>
  </phones>
 </person>
</doc>
*END*

%rules = (
    _default => sub {$_[0] => $_[1]->{_content}},
    bogus => undef,
    address => sub {address => "$_[1]->{street}, $_[1]->{city} ($_[1]-
+>{country})"},
    phone => sub {$_[1]->{type} => $_[1]->{content}},
        # let's use the "type" attribute as the key and the content as
+ the value
    phones => sub {delete $_[1]->{_content}; %{$_[1]}}, 
        # remove the text content and pass along the type => content f
+rom the child nodes
    person => sub { # lets print the values, all the data is readily a
+vailable in the attributes
        print "$_[1]->{lname}, $_[1]->{fname} <$_[1]->{email}>\n";
        print "Home phone: $_[1]->{home}\n" if $_[1]->{home};
        print "Office phone: $_[1]->{office}\n" if $_[1]->{office};
        print "Fax: $_[1]->{fax}\n" if $_[1]->{fax};
        print "$_[1]->{address}\n\n";
        return; # the <person> tag is processed, no need to remember w
+hat it contained
    },
);
[download]

Even though I talked about transforming the data structure there is nothing preventing us from applying the rules as we parse the document as soon as we encounter the closing tag. So we do not have to load the whole document to memory if we don't need it all at once. And even if we do we have a chance to trim it down as we read it and end up with a much smaller data structure.

The rules receive two parameters, the name of the tag and a hash containing the attributes and the content. For leaf nodes the attributes are the tag attributes and the _content is the textual content of the tag, for other tags it's a bit more complex, the data structure contains stuff returned by the rules of the subtags. The rules may return

1. nothing (empty list, undef of empty string) - nothing gets added to the parent's data structure
2. a single string - the string gets appended or pushed to the _content of the parent
3. a single reference - the parent's _content is converted to an array (if necessary) and the reference is pushed there
4. an even numbered list - add the keys (odd items) and values (even items) to the parent's data structure, if the key starts with '@' push the value at the end of the array referenced by the key (without the '@'). The value may be a reference.
5. everything else is an error