As pointed out by others, Mu is the most undefined value there is in Perl 6 and the base root for the immediate child classes (https://docs.perl6.org/images/type-graph-Mu.svg). However, most classes (both built-in and user-defined ones) don't inherit directly from it. Instead, they inherit from Any, which in turns inherits from Mu.
Right. Here's a blog post, which, while not directly about Mu (it's about m, a shell script I wrote), has many excerpts about (the original) Mu (from which Perl6's Mu descends, pun intended, ha ha) near the end - some may find them interesting:
m, a Unix shell utility to save cleaned-up man pages as text:
m, a Unix shell utility to save cleaned-up man pages as text:
https://jugad2.blogspot.com/2017/03/m-unix-shell-utility-to-...