The idea of 6 as an island of stability and 5 as a moving target is exactly backwards.
And Perl 5 has had fewer breaking changes than any of its popular scripting siblings... Ruby (1.8 -> 1.9 anyone?), Python (oh, yes, let's talk about 2 vs 3, shall we?), or PHP (in which build flags introduce breaking changes, let alone stuff that's come with some 5.x dot release).
> So what are the metrics to port Perl 5 to 6? I have no idea, nobody really does. What does it cost to move 2 million lines of Perl 5 to Perl 6?
It doesn't matter what it'd cost at this point because there's no reason to do it, nor is there any visible time on the horizon at which you'd need to do it. Like the GP said, 5 is production, 6 is where people experiment with language features, and 5 has shown that it has plenty of future through regular stable releases and feature enhancements since 5.10 (released 7 years ago!).
> Every technical decision maker ruminating on Perl has gone through this, and they've decided to not go with Perl for these kinds of reasons.
Every technical decision maker worried about the Perl 5->6 transition over the last few years is someone who didn't actually do the kind of diligence a good technical decision maker should do, and hopefully that's a one-off mistake rather than indicative of the general quality of their judgment.