Assuming fetchrow_arrayref is returning multiple rows but each row has one column which is your case SQL beginning with a comma already then why can't you just do:

my $case = $case_query1->fetchall_arrayref();
my $casestr = join(q{}, @{$case});
.
.
.
my $case_stmt3 = "create table case_st_data1 as                     (S
+ELECT
                     rd,
                     store_number
                     $casestr
                  FROM
                    test_ptw_bottom_10
                  where
                    week_end_date in ('19-SEP-09','26-sep-09')
                  group by
                     rd,
                     store_number)";
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I appreciate your help. I was able to get it to work! Thanks!