This also currently lives on GitHub at https://github.com/PDLPorters/pdl/blob/master/Example/doc-pp. It helps me see at a glance what PP parameters get data from each other, but the principle should be widely applicable:

use strict;
use warnings;
use PDL::PP;
use Graph;
use GraphViz2;

my $g = Graph->new; # should really be hypergraph but GraphViz2 not do
+ yet
for my $r (@{$PDL::PP::deftbl}) {
  for my $t (@{$r->{targets}}) {
    $g->add_edge($t, $_) for map s/_//gr, @{$r->{conditions}||[]};
  }
}
my ($fmt) = $ARGV[0] =~ /\.([^.]+)$/;
$g->set_graph_attribute(graphviz=>{graph=>{rankdir=>'LR'}});
GraphViz2->from_graph($g)->run(format=>$fmt,output_file=>$ARGV[0]);

=head1 NAME

doc-pp - Generate graph of pp_def key dependencies with graphviz

=head1 SYNOPSIS

  doc-pp deps.svg

=head1 DESCRIPTION

Uses L<Graph> and L<GraphViz2> to visualise the dependencies between
keys in L<PDL::PP/pp_def>.
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