The Raku programming language is the sixth major revision of Perl.
It was designed in order to tackle with the caveats that Perl had accumulated during its now long history. Those caveats were mainly due to a requirement of backward compatibility of successive versions of Perl. That's why Raku is the first version of Perl that is not backward-compatible.
Raku does not replace Perl. It is rather a sister language or, as some like to put it, the R&D branch of Perl. To some degree, Raku is to Perl what C++ is to C. Although C++ is very successful programming language, it has not in any way replaced C.