my $document = new My::String( read_file('doc.txt') );
  my @sections = $document->split( qr/^------\n/ );
  my @markers = $sections[2]->match( qr/{{\w+}}/ );
  
  foreach $marker (@markers) {  # $marker will be a My::String object
    my $old = $marker->text();
    
    my $new = '';
    # ...
    # Complex code to generate replacement text, depending on the
    # exact contents of $old, goes here
    # ...
    
    $marker->setText( $new );  # change propagates to $document
  }
  
  write_file( 'doc.txt', $document->text() );