$sql = <<EOF;
SELECT "tblForwardMail"."ForwardMailID", "tblFilterMail"."FilterMailEmail"
            FROM "tblForwardMail"
            LEFT JOIN "tblFilterMail" ON "tblForwardMail"."ForwardMailID"="tblFilterMail"."ForwardMailID"
            WHERE "tblForwardMail"."ForwardMailTo"=?
EOF
#    $sth = $dbh->prepare($sql);
#    $sth->execute( $local_address );
#    while (@row = $sth->fetchrow) {
#        push(@filter, $row[1]);
#    }
#
# But it seems like what you want next is just:
(undef,$filter_address) = $dbh->selectrow_array($dbh,{},$local_address);
# make sure to pre-declare $filter_address