Nice. It does have negative one side effect, though.
If you are reading from some other file (say <STDIN>
and you reference $pos, the tell or seek will change perl's idea of the most recent input file to F. This
will mess up the "<STDIN> line n"
in any error/warning message, and also change what handle the $. var is looked up in.
To fix this, put a local $.; in STORE and FETCH. (For some reason, this then triggers some warnings
from STORE when $pos is localized. Fix by saying seek($pos||0,0)).
If you are downvoting this for other than lack of interest, thanks for sending me a message or reply to let me know why so I can do better (or just stay quiet) next time.