Your tutorial is easy to understand, that is good for someone trying to learn without having much Perl in their background. The one thing I thought of, which may cause a problem for beginners, is your use of @a for the example array. It's a minor nitpick, but it lends itself to the use of $a in the dereferencing, and that can make beginners erroneously think $a is a good choice as a variable in code. Of course, it's not $a in scalar context, but beginners usually don't know that.

Oh well, maybe hitting the $a and $b glitch with sort is just one of the rights of passage of every Perl programmer. Maybe you could just mention the $a $b sort conflict, so when new Perl programmers read your tutorial, they will be aware. Or, use @ar for examples?