If the external program can be persuaded to print to STDOUT instead of a file, you can do:

open PRG,"/some/prog|" or die "Cannot fork prog";
while (<PRG>) {
    # do something with the line in $_
}
close PRG;
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Otherwise, you could try a test for the file size every X seconds, and see if it changes, and assume it's done when the file size doesn't change anymore. (this is an heuristic, not a 100% reliable way of doing this)

Don't know if it's possible to see open files over NFS, does any monk have some insight?

Joost.