my $sql = qq/ SELECT a.somedata1,
                         a.somedata2,
                         a.somedata3,
                         [...]
                    FROM table1 a,
                         table2 b,
                         table3 c
                   WHERE a.somedata4 > ?
              /;

      $sql .= qq/    AND a.somedata5 <= ?
                     AND [...]
              / if some_perl_code_condition1;

      $sql .= qq/    AND b.somedata6 >= ?
                     [...] 
              / if some_perl_code_condition2;

      $sql .= qq/ORDER BY somedata7
              /;