G'day perlpearls,

Welcome to the monastery.

I agree with derby's comments above. I was interested to see how this might be achieved with MIME::Lite so I ran some tests. Having now seen your subsequent response, I realise that this isn't what you were after; however, someone might find it useful.

Here's my test code:

$ perl -Mstrict -Mwarnings -e '
    use utf8;
    use MIME::Lite;
    package MIME::Lite::IO_Handle {
        no warnings "redefine";
        sub print {
            my $self = shift;
            binmode $$self => ":utf8";
            print {$$self} @_;
        }
    };
    MIME::Lite::->new(
        To => "ken",
        From => "ken",
        Type => "text/plain; charset=utf8",
        Subject => "\x{263a} Happy days!",
        Data => "Testing 1 2 3",
    )->send;
'
[download]

Notes:

• use utf8; — because there's a Unicode character in the source code.
• package MIME::Lite::IO_Handle { ... } — to deal with message "Wide character in print at /.../MIME/Lite.pm line 3164 ...". The only change to the original method is the addition of binmode $$self => ":utf8";.
• \x{263a} — that's a smiley face. (The sad face \x{2639} also worked.)

Here's the mail that arrived:

$ mail
Mail version 8.1 6/6/93.  Type ? for help.
"/var/mail/ken": 1 message 1 new
>N  1 ken@ganymede.localho  Thu Jun  6 11:21  19/638   "☺ Happy days!"
?
Message 1:
From ken@ganymede.localhost  Thu Jun  6 11:21:44 2013
X-Original-To: ken
Delivered-To: ken@ganymede.localhost
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset=utf8
MIME-Version: 1.0
X-Mailer: MIME::Lite 3.029 (F2.84; T1.38; A2.12; B3.13; Q3.13)
Date: Thu, 6 Jun 2013 11:21:44 +1000
To: ken@ganymede.localhost
From: ken@ganymede.localhost
Subject: ☺ Happy days!

Testing 1 2 3

? 

I'm fairly certain the subject line is text only. If you're seeing images in email subject lines, chances are it's just a unicode character.

Update: and there's probably some email client somewhere that will turn emoji codes to the proper unicode representation.

Thanks for the response.

Probably I should have made myself more clear. I was thinking in the lines of favicon which you see in the browser infront of the url.

SMTP, the standard email protocol, does not specify support for anything like a "favicon". While there may be some email clients which add it as an extension, they are exceptions to the rule. Even if you were to arrange for your image to appear in the subject line on one mail client, the vast majority will not display it at all (indeed, many aren't even capable of displaying an image - text-only mail readers are still around) and, even among those which display it, most would probably display it elsewhere.