I'd like to make a minor tweak to my solution. There's a sixth parameter "size" for binary_search, which is the point at which it reverts to a linear search, and it's defaulted to 512. That means the small @F array will be done with a linear search, after all.

I set the size down and did one more optimization, as follows:

my $lo_pos = binary_search(0,$#F,$lo,\&reader,\@F,2);
my $hi_pos = binary_search($lo_pos,$#F,$hi,\&reader,\@F,2);
[download]

I added a counter to the reader function and got the following output:

Did 6 probes for low
Did 6 probes for high
32,33,34,35
Did 6 probes for low
Did 7 probes for high
50,51,52,53,54,55,56,57,58,59,60,61,62,63,64
[download]

This is compared to a total of 78 probes the original way.