test.pl

use v5.26.0;
use warnings;
use Email::MIME;
use Email::Simple;

{
  my $text = <<~'END';
  Header-A: Foo
  Subject: =?UTF-8?B?VGhpcyBpcyBhIHJlYWxseSBsb25nIGxpbmUgdGhhdCB3aWxsIGVuZCB1cCBnb2luZyBvdmVyIDc4IGNoYXJhY3RlcnMgd2hlbiBlbmNvZGVkIGZvciBVVEYtOA==?=
  Header-B: Bar

  Body
  END

  my $email = Email::Simple->new($text);
  print $email->as_string;
}

say '-' x 70;

{
  my $email = Email::Simple->create(
    header => [
      'Header-A'  => 'Foo',
      'Subject'   => '=?UTF-8?B?VGhpcyBpcyBhIHJlYWxseSBsb25nIGxpbmUgdGhhdCB3aWxsIGVuZCB1cCBnb2luZyBvdmVyIDc4IGNoYXJhY3RlcnMgd2hlbiBlbmNvZGVkIGZvciBVVEYtOA==?=',
      'Header-B'  => 'Bar',
    ],
    body => 'Body',
  );

  print $email->as_string;
}

say '-' x 70;

{
  my $email = Email::MIME->create(
    header_str => [
      'Header-A'  => 'Foo',
      'Subject'   => "\N{CAT FACE}" . 'This is a really long line that will end up going over 78 characters when encoded for UTF-8',
      'Header-B'  => 'Bar',
    ],
    body => 'Body',
  );

  print $email->as_string;
}