Hmmm ... not that i know of but what's your concern? Is the data computed on the fly or in files? If its files and you're worried about memory consumption, don't be. Using MIME::Lite, a snippet such as this:

#!/usr/bin/perl -wd
#
use MIME::Lite;

### Create a new multipart message:
$msg = MIME::Lite->new(
   From    => 'foo@bar.com',
   To      => 'bar@foo.com',
   Subject => 'A message with 2 parts...',
   Type    => 'multipart/mixed'
);

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

$msg->attach(
   Type     => 'text/plain',
   Path     => 'baz.txt',
   Filename => 'baz.txt',
   Disposition => 'attachment'
);

$msg->send;
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is pretty efficient. The file is not processed until the send method is called and then it's pushed across the sendmail pipe (or whatever mailer your using) in 2K chunks

update: looking at the MIME::Lite code, it would be trivial to implement a callback feature. You need to modify the build method to not reject the key and the print_simple_body method to utilize the callback. Then you could do something like this:

$msg->attach(
   Type     => 'text/plain',
   Path     => 'foo.txt',
   Filename => 'foo.txt',
   Disposition => 'attachment',
   Callback => \&hashit,
);

$msg->send;

sub hashit {
   print STDERR "#";
}
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