Your solution also breaks down if there is punctuation in the file. (OS HP-UX 11.0)

File

This is a test file. How many unique words are in this
file? Do you know? Does the file contain more than
ten words?
[download]

Results

   1
   1 a
   1 are
   1 contain
   1 do
   1 does
   1 file
   1 file.
   1 file?
   1 how
   1 in
   1 is
   1 know?
   1 many
   1 more
   1 ten
   1 test
   1 than
   1 the
   2 this
   1 unique
   1 words
   1 words?
   1 you
[download]

Update: Changed the test file.