I see many pitfalls for a General Conversion API, or rather only a single, as it will be either too generic, because it has to accomodate everything, or too special for its title.

The pipelines I know are:

• AxKit - XML::SAX::Machines - an XML transformation pipeline
• My homegrown data transformation and reconcilation engine (written in Python for work). It takes input files, and converts them into hash-lists, the single filters operate on these hashes or the whole list, and the output is a report in the end. It has evolved far enough so that a reconcilation report can now be written in a mostly declarative style.
• A realtime movie/animation transformation engine, much like Adobe AfterEffects, but realtime. This is a toy idea I tinker with. Here, the problem would be that the consumer/producer relation is not entirely clear and must be arbitrated somehow, as you have a target framerate and a (different) framerate at which every producer works, and the arbitrator must make good decisions about where to spend the CPU resources, and every filter involved has to at least export some metadata on how it intends to spend resources allocated to it.

As long as you reduce your problem domain close enough, you can keep most of the meta-data problem out of your house, as the meta-data is common to all filters involved.

As soon as you go "general", your filter API can be reduced to the following interface:

  sub filter::can_connect_to {
    my ($self,$input) = @_;
    return 1;
  };

  sub filter::connect_output {
    my ($self,$next_filter) = @_;
    # ... do magic
  };

  sub filter::on_input_connect {
    my ($self,$input) = @_;
  };

  sub filter::process {
    my ($self, $data) = @_;
    # ... do magic
  };

  sub filter::describe {
    return "Magic filter";  
  };
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This hides all the "arbitration magic" in the connect and on_connect methods, where both filters have to decide on something they can do to each other... I think that without a specific goal, you won't get far with a general concept.

perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web
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