There is nothing from stopping you...
Here are a few things that would hold me back:
- It would be hard to find others who also want to work on such a project (cause, as you say, 'why?')
- With a minuscule team of programmers my development speed will likely be slower than that of the mainstream C perl, therefore I'm writing towards a target that's moving further away all the time.
- Even if I try to write towards a fixed and existing version of perl5 I'll likely not be finished in my lifetime (and if I do nothing but hack on Perlperl all day my lifetime will be even more finite that it is anyway)
You're right, there's nothing in the design of Perl that makes this task impossible in theory, but I'd be willing to bet all the money I own that it is impossible in practice. (Note to prospective punters: it's not very much money :-)
Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it. -- Brian W. Kernighan