I admit I am not very familiar with unicode and asian fonts. I do remember, however, that when I build and install Tk, one of the tests it runs thru is to fill up a window with alot of asian characters, even though I don't have any asian fonts installed. It probably does it just like you did, specifying the unicode characters with Encode? One of the test scripts in the root of the Tk source tree is called "chfont" and it displays asian characters.

use strict;
use Tk;
use Encode qw(decode);

my $face = (@ARGV) ? shift : 'fixed';

my @headers = <DATA>;

my $mw = MainWindow->new;

foreach my $text (@headers)
 {
  my $str = decode('MIME-Header' => $text);
  $mw->Label(-text => $str, -font => [$face,14])->pack;
 }
MainLoop;

__DATA__
From: =?Big5?B?qfq8dw==?= <tukb122@ms12.url.com.tw>
Subject:=?big5?Q?=A9_=A4b=A5=D8=BF=FD=A7=F3=B7s!!?=
Subject: =?Big5?B?o7Ck+S4uLi4uLi4uLi4uLi4uLi4uLg==?=
From: =?Big5?B?tW6//bdqtE2k3sC6?= <twpalm@pchome.com.tw>
Subject: =?Big5?B?rf6xeqq6uvSvuLVuv/23arRNpN7Aug==?=
Subject:=?big5?Q?=A7K=B6O=A5=F4=A5=B4=A4=A4=B4=E4=AA=F8=B3~=B9q=B8=DC?
+=
Subject:=?big5?Q? =A4u=A7@=B2=D6=A4F=A4@=A4=D1=BA=D6=A7Q=B0e=B5=B9=A7A
+  ^0^  =A6A=A6=B8=B7P=C1=C2=BE=E3=B2z=B3o=A5=F7=B8=EA=AE=C6=AA=BA=A4H
+!!!    ?=
Subject:=?big5?Q? =A4u=A7@=B2=D6=A4F=A4@=A4=D1=BA=D6=A7Q=B0e=B5=B9=A7A
+  ^0^  =A6A=A6=B8=B7P=C1=C2=BE=E3=B2z=B3o=A5=F7=B8=EA=AE=C6=AA=BA=A4H
+!!!    ?=
Subject:=?big5?Q? =A4u=A7@=B2=D6=A4F=A4@=A4=D1=BA=D6=A7Q=B0e=B5=B9=A7A
+  ^0^  =A6A=A6=B8=B7P=C1=C2=BE=E3=B2z=B3o=A5=F7=B8=EA=AE=C6=AA=BA=A4H
+!!!    ?=
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