I can't comment on the efficiency of it in a high-volume environment, but I've been using Mail::Box, and its handy Mail::Message class to parse out a messages coming from a procmail script. It's very versatile in dealing with encodings, attachments, etc. and that versatility has been worth the inefficiency of such a heavy object for my purposes. Here's a snippet from my script:

#!/usr/bin/perl -w
use strict;
use Mail::Message;

<>; # toss the mbox format From line
my $msg = Mail::Message->read(\*STDIN);

# (process the message)

# write it back to STDOUT
print $msg->head->createFromLine;
$msg->print;
exit;
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