open(FILE, "<file") or die "error opening $!";
my $buf='';
my $leftover='';
while(read FILE, $buf, 4096) {
  $buf = $leftover.$buf;
  my @lines = split(/\n/, $buf);
  $leftover = ($buf !~ /\n$/) ? pop @lines : "";
  foreach (@lines) { 
    # process one line of data
  }
}
close(FILE);