Sure, embedding Perl in gThumb is an option, but it seems a bit hackish and fraught with peril (i.e., packaging and maintenance problems).

A perl server calling exiftool libs would work, but would duplicate a lot of code in the CLI tool, causing maintenance/tracking pain. But at least the c and perl would be cleanly separated.

Adding it into the standard exiftool CLI tool seems to be the most elegant (add a --daemon option). Maybe it introduces cross-platform issues, I don't know. It would let other similar programs call exiftool in a speedy manner, though.

-Mike