Perl 11 is an effort to make Perl 5 pluggable at the following levels:
This will open up the doors to many kinds of language / technology experimentation, without endangering the existing Perl 5 / CPAN code bases that we depend on every day.
Pluggable VMs would be parrot, p2, JVM or .NET running Perl5 and Perl 6 code. 5 + 6 == 11!
The following projects are important in reaching the vision of Perl 11:
A Restricted Perl by Will Braswell which translates a low-magic subset of Perl 5 into C/C++ using Inline::C and Inline::CPP
cperl is an improved variant of perl5, running all of perl5 and CPAN code. With many perl6 features, just faster. Faster than perl5 and perl6. It is stable and usable, but still in development with many more features being added soon.
p2 by Reini Urban was the attempt to be a fast, small perl11 (5+6 and more) using the potion vm.
potion is the VM behind p2, a small fast language, written by _why's, based on lua, io and others.
MoarVM is basically a freshly boostrapped parrot, with locking threads and a sixmodel MOP, supporting rakudo perl6.
"Not Quite Perl" is current abstraction layer between perl6 and parrot, with parrot and jvm backends, and moarvm and js coming along.
Perlito by Flavio Glock is a source level perl5/6 to perl5/6 and other backends compiler.
perl6/std with the viv metacompiler contains the canonical Perl6 grammar and now also a Perl5 grammar. Written in perl6, interpreted and compiled in perl5 (via viv).
gperl by Masaaki Goshima compiles to heavily optimized LLVM and is ~3x faster than p2. Currrently it is being decoupled to different perl5 Compiler modules.
Seis is a project by Tokuhiro Matsuno to try some perl6 extensions in perl5. pviv by is a perl6 grammar written in greg, which compiles to an AST and will be used as perl6 grammar for p2.