You can wrestle some information out of XML::Twig: you need to use the load_DTD option to read the external DTD, then get the list of elements in the dtd and their models using the model method. It is not perfect, and you then have to do some (simple) post-processing to get just the sub element names.

For example:

#!/usr/bin/perl -w
use strict;
use XML::Twig;

my $t= XML::Twig->new(load_DTD => 1, error_context => 1)->parsefile( '
+test.xml');

my @elts= $t->model;
print "all elements: ", join( ', ', @elts), "\n";

foreach my $elt (@elts)
  { my $model= $t->model( $elt);
    print "model of $elt: $model\n";
    my @subelts= grep { $_ && $_ ne 'PCDATA'} split( /[^\w:-]/, $model
+);  # keep only element names
    # if you want an array of unique element names you then need this
    my %subelts= map { $_ => 1 } @subelts;
    @subelts= sort keys %subelts;
    print "elements in $elt: ", join( ', ', @subelts), "\n";
  }
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Does this help?