use FileHandle; # Maybe should be File::IO ?

if ( -t STDOUT ) {
  STDOUT->autoflush(1);
}

# OR: without FileHandle, save some RAM at the expense
# of legibility:

# braces keep "my" vars local here.
{
  # Save the currently select:ed filehandle!
  my $saved_selected_fh = select STDOUT;
  if ( -t ) {
    ++$|; # turn on "autoflush"
  }
}