my @userids = ('XD4555', 'JJKKKK');

my $sth = $dbh->prepare("SELECT snd.EMAIL FROM FUSERS as m 
                         JOIN USERS as snd ON snd.USERID = m.USERID
                         WHERE m.USERID_IM IN (?)");
$sth->execute(\@userids);
my $mails = $sth->fetchall_arrayref();