I have to modify my code quite frequently. When I start using perl 6, I'm not going to want to program in perl 5&6. I have a hard enough time switching between perl, java, and c# ($,;,#,@,/*,// missing when their needed, and there when their not). Usually I'll change one object, or a sub. When this comes around, I'll have to convert an entire program at once, unless they have some funky way of having perl 5&6 code in the same program (although that would probably be bad form).