You're getting there. Now you are returning the index if the word was found and undef if it wasn't, and you get a warning when you compare undef == 0. You could change your if statement to if ( ! defined $index ). Your words not being found may be because you should be doing while ($low <= $high). The way you have it, you are quitting when you've narrowed down the range to one element instead of checking that element.

Looking at the "Mastering algorithms with perl" O'Reilly Book, it turned out that I had to do $high = $cur - 1 and not +1, to adjust $high.

Sorry, my mistake.