> Actually all attempts to write a Perl 6 compiler in Perl 5 have shown that it's not a very good idea.

It's not at all an attempt to discuss Perl 6, I wouldn't presume to second guess the developers on that topic. It's really just an exposition of the more fundamental ideas of programming languages in general, particularly lexical closures and continuations, and the interesting places you can get to if you follow those leads. Really just a jumping off point for people who might like to explore further.

I must stress that this is not a book about perl, it just happens to use perl because it is a convenient language and has a wide user base (and because I like Perl.)

perl -e 'print sub { "Hello @{[shift->()]}!\n" }->(sub{"World"})'
[download]

...that jogs the memory.... i do remember the initial promo on Perl6, where an assembly language of sorts, called Parrot, was going to be the base language for all interpreters, like Perl, Python, Ruby, etc.

...whatever happened to the dream of the Grand Unified Language ? .. :-)

i do remember the initial promo on Perl6, where an assembly language of sorts, called Parrot

Parrot is a virtual machine; the assembly language is called PIR (Parrot Intermediate Representation). But it's not essential for language interaction. The important concept is that parrot knows about common concept in dynamic programming languages (like subs, methods, strings, integers, other numbers, conversion between types, namespaces), and thus allows you to call methods on foreign objects.

Since Perl 5 was never ported to Perl 6, I don't see how parrot relates to the statement that Perl 5 is a poor assembly language.

...whatever happened to the dream of the Grand Unified Language ?

That's still alive, and in Rakudo you can eval() Ruby code, and load Ruby libraries - to the extend that they run on the ruby compiler for parrot, which is quite limited right now.