Hi gibus,
The code you posted won't even compile for me (on Windows):

Malformed UTF-8 character: \xe9\x27\x20 (unexpected non-continuation b
+yte 0x27,immediately after start byte 0xe9; need 3 bytes, got 1) at t
+ry.pl line 11.
Malformed UTF-8 character (fatal) at try.pl line 11.
[download]

Seems to work fine for me if I rewrite your script as:

#!/usr/bin/perl

use strict;
use warnings;
use utf8;
use Devel::Peek;
use Data::Dumper;
$Data::Dumper::Useqq = 1;

my $s;

{
 no utf8;
 $s = 'clé';
}

utf8::upgrade($s);

my %hash = (
    $s => 0,
);
my $key = (keys %hash)[0];
Dump($key);
print Dumper($key);

$hash{$s} = 1;
$key = (keys %hash)[0];

Dump($key);
print Dumper($key);

utf8::upgrade($key); # does nothing
Dump($key);
print Dumper($key);
[download]

UPDATE: When I initially posted this rewritten version, my utf8::upgrade($s); was done inside the no utf8{} block - which is rather counter-intuitive, to say the least.
So I've subsequently moved it outside the no utf8{} block.

UPDATE 2: The output of my modified script:

SV = PV(0x84c2a8) at 0x373100
  REFCNT = 1
  FLAGS = (POK,pPOK,UTF8)
  PV = 0x247ede8 "cl\303\251"\0 [UTF8 "cl\x{e9}"]
  CUR = 4
  LEN = 5
$VAR1 = "cl\x{e9}";
SV = PV(0x84c2a8) at 0x373100
  REFCNT = 1
  FLAGS = (POK,pPOK,UTF8)
  PV = 0x247f4d8 "cl\303\251"\0 [UTF8 "cl\x{e9}"]
  CUR = 4
  LEN = 5
$VAR1 = "cl\x{e9}";
SV = PV(0x84c2a8) at 0x373100
  REFCNT = 1
  FLAGS = (POK,pPOK,UTF8)
  PV = 0x247f4d8 "cl\303\251"\0 [UTF8 "cl\x{e9}"]
  CUR = 4
  LEN = 5
$VAR1 = "cl\x{e9}";
[download]

HTH.

Cheers,
Rob