Shouldn't the question about syntax rules be more like: "I want to spexify feature x. How can I specify that in perl syntax" ? Or is there something you don't like about your syntax now?

aliasing of $a and $b in sort is (to my knowledge) magical. That means its a built-in of the interpreter not available to us mere mortals (at least it was, maybe perl version 5.8 or 5.10 added access to it and I forgot)

Ideas? You might add a default rule (i.e. the syntax to declare a rule as default)? You also might add a mechanism to enable implicit rules (the knowledge how to create a .o file from a .c file...).