#!/usr/bin/env perl
use strict;
use warnings;
use MIME::Lite::TT::HTML;
use Encode;

my %params;

$params{umlaut} = 'fööbär';
Encode::_utf8_on($params{umlaut});

my %options= (
  INCLUDE_PATH => '.',
  ENCODING => 'UTF-8',
);

my $msg = MIME::Lite::TT::HTML->new(
          From        =>  'admin@example.com',
          To          =>  'frank@example.com',
          Subject     =>  'Your recent purchase',
          Template    =>  {
                              text    =>  'revsys.txt.tt',
                              html    =>  'revsys.html.tt',
                          },
          TmplOptions =>  \%options,
          TmplParams  =>  \%params,
);

binmode STDOUT, ':utf8';
print "Should be: ", $params{umlaut},$/;
print '-' x 79,$/;
print $msg->as_string;

##</code><code>##

revsys.txt.tt:
text: [% umlaut %]

revsys.html.tt:
<h1>[% umlaut %]</h1>


##</code><code>##

Should be: fööbär
-------------------------------------------------------------------------------
Content-Transfer-Encoding: binary
Content-Type: multipart/alternative; boundary="_----------=_158080661890"
MIME-Version: 1.0
X-Mailer: MIME::Lite 3.031 (F2.85; T2.17; A2.21; B3.15; Q3.13)
Subject: =?US-ASCII?B?WW91ciByZWNlbnQgcHVyY2hhc2U=?=
Date: Tue,  4 Feb 2020 08:56:58 +0000
To: frank@example.com
From: admin@example.com

This is a multi-part message in MIME format.

--_----------=_158080661890
Content-Disposition: inline
Content-Transfer-Encoding: 7bit
Content-Type: text/plain; charset=us-ascii

text: fbr
--_----------=_158080661890
Content-Disposition: inline
Content-Transfer-Encoding: 7bit
Content-Type: text/html; charset=us-ascii

<h1>fbr</h1>
--_----------=_158080661890--