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)).