how to preserve original line order?

[Roldano Cattoni]

The task should be simple, I want to put in uppercase all the words of a
(large) file.

I tried the following:
 - streaming mode
 - the mapper is a perl script that put each line in uppercase (number of
   mappers > 1)
 - no reducer (number of reducers set to zero)

It works fine except for line order which is not preserved.

How to preserve the original line order?

I would appreciate any suggestion.

  Roldano