I also seem to recall seeing a news item that suggested that this keyword would also be available in Perl 5 as of v5.10?

Indeed it will. It is implemented as a weak keyword, thus you do have to write

  use feature 'say';
[download]

... to activate it. A search revealed that there were a number of modules on CPAN (predating the existence of Perl 6) that already define a routine named 'say'. Adding it as a regular keyword would have broken them, as well as unknowable amounts of code living behind firewalls that the porters could never know about.

The Perl development team places a lot of importance in not breaking existing code (even if it means you have to resort to the above gymnastics in new code). If I remember correctly, a sitecustomize.pl program (run at the start of every incovation implicitly) will allow you to set this once and for all, for all code run on a given host.