There isn't one.

Perl interally uses a bytecode representation, but the only definitive definition of it is in the sourcecode.

What you likely want is the documentation in perlguts, try perldoc perlguts for that. Beyond that both Devel::Peek and B are useful modules for learning about the innards of Perl, as is reading the source-code, etc.

Depending upon what kind of project this is for, you may also want to search for documentation on XS, SWIG, or try the Inline module.

EDIT
Forgot to mention. Creating a real specification is on the wishlist for Perl 6. Initially it sounded like there was a very good chance, but I have not been paying much attention.