use HTML::TokeParser::Simple;
use HTML::Entities;
use File::Copy;

my $new_html  = '';
my $orig_html = shift || die "Usage: $0 some.html";

copy( $orig_html, "${orig_html}.bak") 
    or die "Could not copy ($orig_html): $!";

my $parser = HTML::TokeParser::Simple->new($orig_html);

while (my $token = $parser->get_token) {
  if ($token->is_tag) {
    $new_html .= $token->as_is;
    next;
  }
  $new_html .= encode_entities($token->as_is);
}

open  OUTPUT "> $orig_html" or die "Cannot open ($orig_html) for writing: $!";
print OUTPUT $new_html;
close OUTPUT;