#!/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";
  }