% perl --version
    This is perl 5, version 16, subversion 3 (v5.16.3) built for x86_64-linux-thread-multi
% cat /etc/redhat-release 
    CentOS Linux release 7.3.1611 (Core) 

##</code><code>##

% testmail.pl
    no data in this part at ./testmail.pp line 25.

##</code><code>##

#!/usr/bin/perl -w

use strict;
use warnings;

use MIME::Lite;
use Data::Dumper;

my $msg = MIME::Lite->new();

dumpObject($msg);

$msg->build(to => 'user1 <user@fqdn.org>', from => 'user2 <user2@fqdn.org>');
$msg->build(subject => 'a message subject');

dumpObject($msg);

$msg->attach(
    Type    => 'text/plain',
    Data    => 'This is the body text of the email',
);

dumpObject($msg);

$msg->send;              # <------  'no data' error here

sub dumpObject {
    my ($object) = @_;

    my $output = Data::Dumper->Dump([$object], ['foo']);
    $output =~ s/\$foo = //;

    print $output;
}

##</code><code>##

./testmail.pp
bless( {
    'SubAttrs' => {},
    'Parts' => [],
    'Attrs' => {},
    'Header' => []
  }, 'MIME::Lite' );

##</code><code>##

bless( {
    'SubAttrs' => {},
    'Parts' => [],
    'Attrs' => {
         'content-disposition' => 'inline',
         'content-type' => 'text/plain',
         'mime-version' => '1.0',
         'content-length' => undef,
         'content-transfer-encoding' => '8bit'
       },
    'Header' => [
      [
        'date',
        'Sat, 29 Jul 2017 12:31:40 -0700'
      ],
      [
        'to',
        'user1 <user@fqdn.org>'
      ],
      [
        'from',
        'user2 <user2@fqdn.org>'
      ],
      [
        'x-mailer',
        'MIME::Lite 3.030 (F2.84; T1.38; B3.13; Q3.13)'
      ],
      [
        'date',
        'Sat, 29 Jul 2017 12:31:40 -0700'
      ],
      [
        'subject',
        'a message'
      ]
    ]
  }, 'MIME::Lite' );

##</code><code>##

bless( {
    'SubAttrs' => {
        'content-type' => {
            'boundary' => '_----------=_1501356700259120'
          }
      },
    'Parts' => [
       bless( {
          'SubAttrs' => {},
          'Parts' => [],
          'FH' => undef,
          'Attrs' => {
               'content-disposition' => 'inline',
               'content-type' => 'text/plain',
               'content-length' => undef,
               'content-transfer-encoding' => '8bit'
             },
          'Path' => undef,
          'Data' => undef,
          'Header' => []
        }, 'MIME::Lite' ),
         bless( {
          'SubAttrs' => {},
          'Parts' => [],
          'Attrs' => {
               'content-disposition' => 'inline',
               'content-type' => 'text/plain',
               'content-length' => undef,
               'content-transfer-encoding' => '8bit'
             },
          'Data' => 'This is the body text of the email',
          'Header' => []
        }, 'MIME::Lite' )
       ],
    'Attrs' => {
         'content-type' => 'multipart/mixed',
         'mime-version' => '1.0',
         'content-transfer-encoding' => '7bit'
       },
    'Header' => [
      [
        'date',
        'Sat, 29 Jul 2017 12:31:40 -0700'
      ],
      [
        'to',
        'user1 <user@fqdn.org>'
      ],
      [
        'from',
        'user2 <user2@fqdn.org>'
      ],
      [
        'date',
        'Sat, 29 Jul 2017 12:31:40 -0700'
      ],
      [
        'subject',
        'a message'
      ],
      [
        'x-mailer',
        'MIME::Lite 3.030 (F2.84; T1.38; B3.13; Q3.13)'
      ]
    ]
  }, 'MIME::Lite' );
no data in this part at ./testmail.pp line 21.

##</code><code>##

    if ( defined( $self->{Data} ) ) {
...
    elsif ( defined( $self->{Path} ) || defined( $self->{FH} ) ) {
...
    else { Carp::croak "no data in this part\n";

##</code><code>##

=item attach PART

=item attach PARAMHASH...

I<Instance method.>
Add a new part to this message, and return the new part.

If you supply a single PART argument, it will be regarded
as a MIME::Lite object to be attached.  Otherwise, this
method assumes that you are giving in the pairs of a PARAMHASH
which will be sent into C<new()> to create the new part.

One of the possibly-quite-useful hacks thrown into this is the
"attach-to-singlepart" hack: if you attempt to attach a part (let's
call it "part 1") to a message that doesn't have a content-type
of "multipart" or "message", the following happens:

=over 4

=item *

A new part (call it "part 0") is made.

=item *

The MIME attributes and data (but I<not> the other headers)
are cut from the "self" message, and pasted into "part 0".

=item *

The "self" is turned into a "multipart/mixed" message.

=item *

The new "part 0" is added to the "self", and I<then> "part 1" is added.

=back

One of the nice side-effects is that you can create a text message
and then add zero or more attachments to it, much in the same way
that a user agent like Netscape allows you to do.

=cut

sub attach {
    my $self = shift;
    my $attrs = $self->{Attrs};
    my $sub_attrs = $self->{SubAttrs};

    ### Create new part, if necessary:
    my $part1 = ( ( @_ == 1 ) ? shift: ref($self)->new( Top => 0, @_ ) );

    ### Do the "attach-to-singlepart" hack:
    if ( $attrs->{'content-type'} !~ m{^(multipart|message)/}i ) {

        ### Create part zero:
        my $part0 = ref($self)->new;

        ### Cut MIME stuff from self, and paste into part zero:
        foreach (qw(SubAttrs Attrs Data Path FH)) {
            $part0->{$_} = $self->{$_};
            delete( $self->{$_} );
        }
        $part0->top_level(0);    ### clear top-level attributes

        ### Make self a top-level multipart:
        $attrs = $self->{Attrs} ||= {};       ### reset (sam: bug?  this doesn't
 reset anything since Attrs is already a hash-ref)
        $sub_attrs = $self->{SubAttrs} ||= {};    ### reset
        $attrs->{'content-type'}              = 'multipart/mixed';
        $sub_attrs->{'content-type'}{'boundary'}      = gen_boundary();
        $attrs->{'content-transfer-encoding'} = '7bit';
        $self->top_level(1);      ### activate top-level attributes

        ### Add part 0:
        push @{ $self->{Parts} }, $part0;
    }

    ### Add the new part:
    push @{ $self->{Parts} }, $part1;
    $part1;
}