my $xml = XMLin($file);
for my $foo (@{$xml->{foo}) {
  #and now we process the $foo
}

=>

$xmltwig->parse( xml_roots => {
  'foo' => \&process_foo
});
sub process_foo {
  my ($xmltwig, $foo_obj) = @_;
  $foo = $foo_obj->simplify();
  #and now we process the $foo
}