Well, to sort of answer one of your questions- after writing, you need to go back to the begining of the file to read it. The cursor (more or less) is left just after where you finished writing. Set the cursor to the beginning of the file again with a seek(FILEHANDLE, 0, 0).

Then, after a read, you'll be ready at the end of the file for the next write. So goes the theory...
Your unpredictable results will be closer to what you expect. Sort of.

Probably you shouldn't be doing this, though.

-><- -><- -><- -><- -><-
All things are Perfect
    To every last Flaw
    And bound in accord
         With Eris's Law
 - HBT; The Book of Advice, 1:7