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This would probably be a lot more useful if it were implemented as a subclass of Tie::File.
Actually, this seems like a job for inside-out objects. I just uploaded to CPAN a new, quick-and-dirty version of File::Marker (0.11) that adds the ability to save/load markers from an external file. It works similarly to what you've described, though you have to do the line-numbering work yourself. On the flip side, it handles the seeking for you and works like an IO::File object (which it subclasses).
Creating the index:
Re-use the index
You could probably extend it to focus on line-numbers instead of marking any type of position in a file or add the various special features people have mentioned. Alternatively -- perhaps even better -- would be to reimplement it using Class::InsideOut, since File::Marker is only really intended to be a teaching module. If I get around to figuring out how to sanely and safely serialize IO::File based inside-out objects with Storable, this could be even easier.
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