STD is Larry Wall's reference implementation of the Perl 6 grammar,
parser generator, and some of the semantic verifier. STD is an
abbreviation of "Standard", as in "Perl6.0.0-STD". It is written in
Perl 6, transliterated into Perl 5 by viv.
It is also used by
various phases of several other compilers (mildew, perlito, elf).
Perl 5 code in Perl 6. Supports Perl 5 modules including XS
modules like DBI. Allows
passing integers, strings, arrays and hashes. Also supports calling methods
on Perl 5 objects from Perl 6 and calling methods on Perl 6 objects from Perl 5.
setting library which defines all of the classes, routines, and terms
available when your script starts. In Perl 5 parlance, it's roughly all the
things you can see in perlfunc, perlop, etc. This part of Perl 6 is
mostly written in Perl 6 (and NQP).