You can't create a new empty object and add data to it.

Sure you can

#!/usr/bin/perl --
use strict;
use warnings;
use MIME::Lite;
my $msg = MIME::Lite->new; ## no data
$msg->build( qw/ to you from me Data Data1 /); ## data
$msg->build( qw/ subject SUBJECT  /)->data('Data2'); ## data
$msg->attach( qw/ Data DATA3 /); ## data
$msg->print;
__END__
Content-Transfer-Encoding: 7bit
Content-Type: multipart/mixed; boundary="_----------=_150136405630120"
MIME-Version: 1.0
Date: Sat, 29 Jul 2017 14:34:16 -0700
To: you
From: me
Date: Sat, 29 Jul 2017 14:34:16 -0700
Subject: SUBJECT
X-Mailer: MIME::Lite 3.029 (F2.84; T2.04; A2.12; B3.14; Q3.13)

This is a multi-part message in MIME format.

--_----------=_150136405630120
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
Content-Type: text/plain

Data2
--_----------=_150136405630120
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
Content-Type: text/plain

DATA3
--_----------=_150136405630120--
[download]

Aha! There's the issue:

% diff foo.pl bar.pl 
7c7
< $msg->build(subject => 'a message subject')->data('Data2');
---
> $msg->build(subject => 'a message subject');

foo.pl -> 0
bar.pl -> 1
[download]

Why is the ->data() required?

Because its written that way?

build returns an object of some kind ... if you dont specify data in constructor, you have to specify it later ...

Notice also data1 is disappeared :)