Email::MIME bug?

gistfile1.prolog

#!/usr/bin/perl

# Using Transport::SMTP, body is grabled with some case.
# With same code,
#   Transport::Sendmail is ok(not garbled)
#   Transport::SMTP is ng(garbled)

use strict;
use Carp;
use warnings;
use Email::MIME;
use Email::Sender::Simple qw(sendmail);
use Email::Sender::Transport::SMTP;
use Email::Sender::Transport::Sendmail;
use utf8;

my @header_str0 = ( # all key & value will be not utf8 flagged (ok with SMPT/Sendmail).
                   'From'    , '"ktat" <ktat@exampl.com>',
                   'To'      , '"ktat" <ktat@localhost>',
                   'Subject' , 'タイトル',
                  );

my @header_str1 = ( # keys(left of '=>') are utf8 flagged. (ng with SMTP)
                   From    => '"ktat" <ktat@exampl.com>',
                   To      => '"ktat" <ktat@localhost>',
                   Subject => 'タイトル',
                  );

my @header_str2 = ( # To value will be encoded, but utf8 flagged. (ng with SMTP)
                   'From'    , '"ktat" <ktat@exampl.com>',
                   'To'      , '"加藤敦" <ktat@localhost>',
                   'Subject' , 'タイトル',
                  );

{ # test of the above @header_str2
  my $x = $header_str2[3];
  my $y = Encode::encode("MIME-Q", $x, 1);
  print $y, "\n"; # "=?UTF-8?Q?=E5=8A=A0=E8=97=A4=E6=95=A6?=" <ktat@localhost>
  print utf8::is_utf8($y), "\n"; # 1
}

my $email = Email::MIME->create
  (
   header_str => \@header_str1,
   attributes => {
                  content_type => 'text/plain',
                  charset      => 'UTF-8',
                  encoding     => '8bit',
                 },
   body_str => '本文',
  );

$| = 1;
warn "\n>>> 1 NG\n";
warn  $email->as_string; # utf8 flagged string(header) + not utf8 flagged string
warn "\n>>> 2 OK\n";
print $email->as_string; # print is ok. I don't know why it is ok. 
                         # it is the reason why Transport::Sendmail is ok (it uses 'print' to pipe)
warn "\n>>> 3 OK\n";
warn $email->__head->as_string; # utf8 flagged string with no wide chars
warn "\n>>> 4 OK\n";
warn $email->body_raw;
warn "\n>>> NG\n";     # as same as 1 ($email->as_string does this)
warn $email->__head->as_string . $email->body_raw; # NG
print "\n";

# use SMTP
sendmail($email,
         {transport => Email::Sender::Transport::SMTP->new
          ( {
             'timeout' => 120,
             'ssl' => 0,
             'allow_partial_success' => 0,
             'host' => '127.0.0.1'
            })}
        );

# use Sendmail
sendmail($email, {transport => Email::Sender::Transport::Sendmail->new});