XML::Filter::Base is a base class for PerlSAX filters that by default
simply passes the event on to the next handler. You can use this to
build very simple filters, for example a filter that lower-cases
all your XML tags:

  package XML::Filter::Lower;
  use XML::Filter::Base;
  @ISA = ('XML::Filter::Base');
  
  sub start_element {
    my ($self, $element) = @_;
    $element->{Name} = lc($element->{Name});
    $self->{Handler}->start_element($element);
  }
  
  sub end_element {
    my ($self, $element) = @_;
    $element->{Name} = lc($element->{Name});
    $self->{Handler}->end_element($element);
  }

or a filter that changes certain elements based on a given hash:

  package XML::Filter::ElementHash;
  use XML::Filter::Base;
  @ISA = ('XML::Filter::Base');
  
  sub start_element {
    my ($self, $element) = @_;
    $element->{Name} = $self->{ElementHash}{$element->{Name}} 
                    || $element->{Name};
    $self->{Handler}->start_element($element);
  }
  
  sub end_element {
    my ($self, $element) = @_;
    $element->{Name} = $self->{ElementHash}{$element->{Name}} 
                    || $element->{Name};
    $self->{Handler}->end_element($element);
  }

If you want a filter that reads XML and writes it out the other end,
check out XML::Handler::YAWriter.