It doesn't look like it. Exegesis 4 does have a lexical constant declaration,

my $n is constant = 11;

but my copy of Perl 5.10 doesn't like it...

This should not be too terribly surprising, since the Exegesis is about Perl 6, not Perl 5. In any case, that is an ancient document, and these days that particular syntax is invalid even in Perl 6, because we've split the concept of constantness in two. As noted elsewhere in this thread, you can't do constant folding unless the compiler actually knows what the constant is going to be at compile time. So we distinguish such true constants from the "fake" constants that are known to stay constant only over a given dynamic scope. Syntactically they are also distinguished:

constant $n = 11;

vs

my $n is readonly = recalculate()

Note that function parameters in Perl 6 default to readonly, not constant, since the actual value can vary from call to call, and hence cannot be constant-folded.

In any case, if you want to quote Perl 6 documents, it's more useful to track the Synopses rather than the Exegeses, and do so at a more up-to-date site. (And, in fact, if you'd gotten your copy of Exegesis 4 from there, it would have had various annotations explaining where the Exegesis was out-of-date.)