APM: changing buffer size to a writable fh

[jeremy]

Hey all,
  I have a need to change the buffer used when writing to a file handle.
The handle to to a flat file.  The script dumps data to it.
..no big woop. 

 More specifically, I need to make sure that none of the  data gets
written to the file until the handle is closed OR until the there is no
more data regardless of how much data gets kept in memory.  I seem to
remember doing this a while back but I don't remember how.  Can someone
point me in the right direction?

thanks,
  Jeremy