parts_add() on multipart/mixed results in two multipart/mixed

[futon88]

It's not clear whether this is an issue, or not, but it certainly caught me by surprise:

my $mixed = Email::MIME->create(
    attributes => {content_type => "multipart/mixed"},
    header_str => [Subject => "Subject"],
);
my $related = Email::MIME->create(
    attributes => {content_type => "multipart/related"},
);
$mixed->parts_add([$related]);
print $mixed->as_string;

Results in:

Subject: Subject
Date: Mon, 25 Aug 2014 10:13:30 -0400
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="1408976010.DFC040.11480"


--1408976010.DFC040.11480
Subject: Subject
Date: Mon, 25 Aug 2014 10:13:30 -0400
MIME-Version: 1.0
Content-Type: multipart/mixed


--1408976010.DFC040.11480
Date: Mon, 25 Aug 2014 10:13:30 -0400
MIME-Version: 1.0
Content-Type: multipart/related

--1408976010.DFC040.11480--

My expectation is that doing a parts_add() with a single new part would be equivalent to parts_set() with the same part, which results in:

Subject: Subject
Date: Mon, 25 Aug 2014 10:15:15 -0400
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="1408976115.Be2FCB00.11602"


--1408976115.Be2FCB00.11602
Date: Mon, 25 Aug 2014 10:15:15 -0400
MIME-Version: 1.0
Content-Type: multipart/related


--1408976115.Be2FCB00.11602--

Anecdotally, these mixed-within-mixed messages appear to render completely blank in Windows Live Mail.

[bbkr]

The workaround I found is to use parts in constructor.

my $related = Email::MIME->create(
    attributes => {content_type => "multipart/related"},
);

my $mixed = Email::MIME->create(
    attributes => {content_type => "multipart/mixed"},
    header_str => [Subject => "Subject"],
    parts => [$related]
);

print $mixed->as_string;

Result:

Subject: Subject
Date: Sat, 31 Oct 2015 09:39:51 +0100
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="14462807910.dDd8E.69946"
Content-Transfer-Encoding: 7bit


--14462807910.dDd8E.69946
Date: Sat, 31 Oct 2015 09:39:51 +0100
MIME-Version: 1.0
Content-Type: multipart/related


--14462807910.dDd8E.69946--