Once? This is the only time there has been a release of a versioned specification for the language compilers can target. That's what has been in development for 15 years! The specification of the actual language. It's like the difference between JavaScript existing as a language for a while and then one day ECMAScript 1 was defined. Apart from Perl 6 was always about the evolution of the spec with various implementations showing the way.

The first major implementation of Perl 6 back in the early/mid 00s was Pugs written in Haskell where Perl 6 picked up a tonne of functional heritage both in the compiler at the time and the language spec. The more recent Rakudo compiler has a lead architect who often works in C# where the compiler and the language spec picked up and redefined a reactive programming concurrency model with lots of nice new syntax like the react/whenever block.

Rakudo Perl 6 and Pugs and the host of other early implementations have been around a while and released often when they were worked on, monthly in the case of Rakudo with the first publicised release in 2010. Rakudo is also releasing a version of the compiler compliant with the newly frozen and released specification.