How about using Unicode::Map?

With it, you can convert the input from utf-8 to utf-16, then perform the XMLin call, and then convert back to utf-8 afterwards (if need be).

For example:

use strict;
use warnings;

use Unicode::Map();
use XML::Simple;

my $string = '<eleve nom="qsdqsd" prenom="Raphaël" classe="Seconde L" 
+adresse="Léon Bourgain" code_postal="3423" ville="machin" pays="Franc
+e" telephone="3421434" />';

# Convert to utf-16
my $map = new Unicode::Map("ISO-8859-1");
my $utf16 = $map->to_unicode($string);

# Recommended to call XMLin using 'eval', to trap errors
my $ref = eval { XMLin($utf16) };
$@ and die "An error occurred in XMLin:  '$@'\n";

# Now you can use '$ref' ...
my $out = XMLout($ref);
print "XMLout => '$out'\n";
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If necessary, when you use the resulting "$ref", you can convert it back to utf-8 with the from_unicode() method of Unicode::Map.