When using sendmail, MIME::Lite opens a pipe to sendmail and does $self->print( \*SENDMAIL );. As you can see from the following, that sends the content of the file (as opposed to having sendmail read the file):

use strict;
use warnings;

use File::Basename qw( basename );
use MIME::Lite     qw( );

my $msg = MIME::Lite->new(
    From        => 'me@myhost.com',
    To          => 'you@yourhost.com',
    Cc          => 'some@other.com, some@more.com',
    Subject     => 'A message with 2 parts...',
    Type        => 'multipart/mixed'
);

$msg->attach(
    Type        => 'TEXT',
    Data        => "Here's the file you wanted"
);

$msg->attach(
    Type        => 'application/perl',
    Path        => $0,
    Filename    => basename($0),
    Disposition => 'attachment'
);

$msg->print();
[download]

Content-Transfer-Encoding: binary
Content-Type: multipart/mixed; boundary="_----------=_125010106643850"
MIME-Version: 1.0
X-Mailer: MIME::Lite 3.01 (F2.74; T1.27; A1.74; B3.07; Q3.07)
Date: Wed, 12 Aug 2009 14:17:46 -0400
From: me@myhost.com
To: you@yourhost.com
Cc: some@other.com, some@more.com
Subject: A message with 2 parts...

This is a multi-part message in MIME format.

--_----------=_125010106643850
Content-Disposition: inline
Content-Length: 26
Content-Transfer-Encoding: binary
Content-Type: text/plain

Here's the file you wanted
--_----------=_125010106643850
Content-Disposition: attachment; filename="a.pl"
Content-Transfer-Encoding: base64
Content-Type: application/perl; name="a.pl"
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--_----------=_125010106643850--
[download]

That means you can safely delete the file as soon as it has been sent if you don't care about delivery problems.