é

##</code><code>##

$  perl -CS -le 'print "na\x{EF}ivet\x{E9}"' 
naïveté

$  perl -CS -le 'print "na\x{EF}ivet\x{E9}"' | 
   perl -C0 -pe 's/(\P{ASCII})/"&#".ord($1).";"/ge'
na&#195;&#175;ivet&#195;&#169;

$  perl -CS -le 'print "na\x{EF}ivet\x{E9}"' | 
   perl -C0 -pe 's/(\P{ASCII})/sprintf "&#x%02X;", ord($1)/ge'
na&#xC3;&#xAF;ivet&#xC3;&#xA9;

##</code><code>##

$  perl -CS -le 'print "na\x{EF}vet\x{E9}"' | 
   perl -CS -pe 's/(\P{ASCII})/"&#".ord($1).";"/ge'
na&#239;vet&#233;

$ perl -CS -le 'print "na\x{EF}vet\x{E9}"' | 
  perl -CS -pe 's/(\P{ASCII})/sprintf "&#x%02X;", ord($1)/ge'
na&#xEF;vet&#xE9;

##</code><code>##

$ perl -CS -le 'print "na\x{EF}vet\x{E9}"' | 
  perl -C0 -pe 's/(\P{ASCII})/"&#".ord($1).";"/ge' | 
  perl -C0 -pe 's/&#(\d+);/chr($1)/ge'
naïveté

##</code><code>##

$ perl -CS -le 'print "na\x{EF}vet\x{E9}"' |
  perl -C0 -pe 's/(\P{ASCII})/"&#".ord($1).";"/ge' | 
  perl -MEncode -C0 -pe 's/&#(\d+);/chr($1)/ge;
                                    $_ = decode_utf8($_, 1); 
                                    s/(\P{ASCII})/"&#".ord($1).";"/ge'
na&#239;vet&#233;

##</code><code>##

$ perl -i.unmangled.by.$$ -MEncode -C0 -pe 's/&#(\d+);/chr($1)/ge; $_ = decode_utf8($_, 1); s/(\P{ASCII})/"&#".ord($1).";"/ge' all*your*broken*files.xhtml

##</code><code>##

#!/usr/bin/env perl
use strict;
use warnings;
use Encode;
die "gimme args" unless @ARGV;
$^I = ".unmangled.by.$$";
while (<>) {
    s/&#(\d+);/chr($1)/ge;
    $_ = decode_utf8($_, 1);
    s/(\P{ASCII})/ "&#" . ord($1) . ";" /ge;
    print;
}