This reply has more to do with parsing messages with attachments rather than sending them, but...

I was recently writing some software that parses messages and separates attachments using the MIME::Entity suite of modules. I specifically had to specify not to save them out to temporary files as in the following code (note the output_to_core):

use IO::File;
use MIME::Parser ();
my $fh = IO::File->new('somemailfile','r');
my $parser = MIME::Parser->new();
$parser->output_to_core(1); # do everything in memory
my $mail_object = eval{ $parser->parse($fh) };
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Sorry this is not necessarily to the point, but I have included it in case you end up doing more than just send the attachments -- which almost always comes soon after being able to send them (ie - save a copy and read it in for later use).

my @a=qw(random brilliant braindead); print $a[rand(@a)];