Well, it's a special case and the ability is going to need to be coded into the parser itself (like the 4 special char codes). One of the older versions specifically the one currently in use on my sites, has just such a routine built in. I will make sure it's done by the time I put out the first release.

As for making a custom plugin, it's very simple.

Lets say you want to add a plugin called foo which takes two attributes called bar and baz, and some meta data.

In the aXML doc
---------------

  <foo bar="hello" baz="world">
     good to be here
  </foo>

  

In your sites private subs module
---------------------------------

my $plugins = {

   foo => sub { 
               my $args = $_[0];  #a hashref
               my $data = $_[1];  #good to be here

               $args->{'tag_name'};  #foo  
               $args->{'bar'};       #hello
               $args->{'baz'};       #world 

               #do some stuff with the args and data here
 
               return 'some result'; 
             }
       };
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In the case where your plugin does not need to return any value to the document, you can just return '<null>'. This is to prevent the parser from throwing a warning about doing a substitution on an empty string, and is automatically removed from the document in the post processing stage.

Also you could create a plugin called "lt" with the following code:

 
  lt => sub { return '<'; }

or 

  lt => sub { '<' }   #golfed :P
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However it wouldn't achieve anything and would result exactly the same as just putting a < into the document to begin with.