The 2019.07 Changelog states:

    Introduced `PERL6_HOME` and `NQP_HOME` which allow to control where
    Perl 6 and NQP look for their installation

This indicates that these are implementation specific environment variables,
since NQP is unique to the Rakudo implementation.  Therefore, the name
PERL6_HOME is essentially incorrect, and therefore the name RAKU_HOME is
also incorrect.

So the environment variable should be called RAKUDO_HOME.