I don't see why unshifting a string onto an array is less simple or more odd than fiddling around with fseek and tell.

If you just want to avoid unshift, you could leave your array intact instead of shifting off it and just remember the line you are (i.e. the array index). Then going back to the previous line is done with $pointer--

If it is about seeking into the middle of a line then you could slurp the whole file into one single string and use substr or a regex (which is still a lot faster than any file access on files of normal length). Or split lines at "interesting" points (use splice to add the additional line into the array) to make sure that the point you want to go to is always at the beginning of a line.