#!/bin/perl -w
use strict;

use XML::Twig;
my $t= XML::Twig->new;
$t->parse( \*DATA);

# tag all matching strings with <a class="xcode">$1</xcode>
# returns the list of newly created elements
my @xref=$t->root->mark( qr/(@\w+)/, 'a', { class => 'xcode'});
foreach my $xref ( @xref)
  { $xref->set_content(  substr( $xref->text, 1) );  # remove the leading @
    $xref->set_att(  href => $xref->text . ".html");
  }
$t->print;

__DATA__

<doc>text with a @reference to whichever @document I want</doc>