os> for (( i=1; i<=99999; i++ )) do printf "1%05d;2%05d;3%05d;4%05d;5%05d;6%05d\n" $i $i $i $i $i $i; done > file.csv
os> cat filex.perl 
#!/usr/bin/perl -n

print "$1;$3;$5\n" if (/^(\w+);(\w+);(\w+);(\w+);(\w+);(\w+)/)

sql> create table ext2 (
  f1 number,
  f3 number,
  f5 number
) organization external (
  default directory oracle_tmp
  access parameters (
    records delimited by newline
    preprocessor 'filex.perl'
    fields terminated by ';'
    ( f1, f3, f5)
  )
  location ('file.csv')
);

sql> declare
   t timestamp with time zone;
   n number;
begin for i in 1..3 loop
   n := 0;
   t := systimestamp;
   for e in (select * from ext1) loop
      n := n + e.f3;
   end loop;
   dbms_output.put_line('nopreproc: '||(systimestamp - t)||' '||n);
   n := 0;
   t := systimestamp;
   for e in (select * from ext2) loop
      n := n + e.f3;
   end loop;
   dbms_output.put_line('preproc..: '||(systimestamp - t)||' '||n);
end loop; end;
/

nopreproc: +000000000 00:00:00.468283000 34999650000
preproc..: +000000000 00:00:00.960613000 34999650000
nopreproc: +000000000 00:00:00.495077000 34999650000
preproc..: +000000000 00:00:00.984496000 34999650000
nopreproc: +000000000 00:00:00.483021000 34999650000
preproc..: +000000000 00:00:00.981991000 34999650000