my $STMT = $DBH->prepare(q{
    SELECT SND.firstname as snd_fname, SND.lastname as snd_lname,
           REC.firstname as rec_fname, REC.lastname as rec_lname
    from MSG as M
    -- We'll use SND as the alias for the sender
    join USERS as SND on M.msg_from = SND.usrid
    -- and REC as the alias for the recipient
    join USERS as REC on M.msg_to   = REC.usrid
    where M.msg_id = ?
});

# Fetch the data for message 5
my $msg_id = 5;

$STMT->execute($msg_id);
while (my $row = $STMT->fetchrow_hashref) {
    # do stuff with the data
}