According to the MIME-tools documentation (specifically the description), the whole bundle can create and parse even nested multipart messages.

I bet the syntax would be something like:

my $top = MIME::Entity->build({ To => 'you@there.pl' });
$top->attach(MIME::Entity->build({ Content-type => 'multipart/alternat
+ive; boundary="abc123---" });
[download]

... but I don't have this installed, I haven't tried it, and my house smells like cleaning fluid.

So give it a shot and I'll refund your money if it doesn't work.