use XML::Rules;

my $parser = XML::Rules->new(
  style => 'filter',
  rules => {
    article => sub {
      my ($tag,$attr) = @_;
      $attr->{_content} =~ s/EM/E/g;
      return $tag => $attr;
    }
  }
);

$parser->filterfile( 'a.xml', 'b.xml');