"Teaching Perl" or even "Teaching Computer Science with Perl." It's the book I'd write if A) I had an extra 1,000 hours in the year and B) my Perl were at a higher level than I could get without another extra 1,000 hours.
It would not sell well, but it would fill a wonderful niche, be good for advocacy, and could probably support a premium price. If it had a companion of "The Perl Workbook" or something for classes then those might take up the sales slack.
I would probably publicly attack such a tome unless
$its_Perl > $competent
$its_instructional_value >= $its_Perl