open FILE, "file" or die "can't open file: $!";
$position{1} = 0;
while (<FILE>) {
  # $. holds line number of file being read from
  $position{$.+1} = tell(FILE);  # record byte offset

  # do stuff with $_
  # access a previous line by doing:

  if (CONDITION) {
    seek FILE, $position{$LINE_NUMBER}, 0;
    $data = <FILE>;
    # ...
    # when done, get back to where you were
    seek FILE, $position{$.+1}, 0;
  }
}
close FILE;