Encode::decode_utf($str, 1) bug?

gistfile1.pl

use Data::Dumper;
my $x = {1 => "ichi", 2 => "ni"};
warn Dumper $x;
{
    use Encode;
    my $decoded = Encode::decode_utf8($x->{1}, 1);
    local $x->{1} = $decoded;
    warn Dumper $x;
}
warn Dumper $x;

# $ perl -MEncode -E 'say $Encode::VERSION'
# 2.42
###### output
# $VAR1 = {
#           '1' => 'ichi',
#           '2' => 'ni'
#         };
# $VAR1 = {
#           '1' => 'ichi',
#           '2' => 'ni'
#         };
# $VAR1 = {
#           '1' => '',
#           '2' => 'ni'
#         };

使いかたの問題だった。
checkに Encode::LEAVE_SRC を渡せばよい