I find it charming. It doesn't need to be the way that particular special variable is done, but twigils are a thing in Perl 6, and I think this is a reasonable use of them.
You still have to check the docs for the dunder variables in Python. Having the same special character convention (like __var__) doesn't magically make me aware of what they all are and what they all mean. And, I've done enough Python programming to know it's a crapshoot to google for them (just as it's a crapshoot to google for sigils and twigils in Perl). You just have to know where to look for the right docs or know who to ask for where to look for the right docs (perldoc perlvar, in the case of Perl...I assume there's something similar for Perl 6, and I don't remember what it is for Python).
You still have to check the docs for the dunder variables in Python. Having the same special character convention (like __var__) doesn't magically make me aware of what they all are and what they all mean. And, I've done enough Python programming to know it's a crapshoot to google for them (just as it's a crapshoot to google for sigils and twigils in Perl). You just have to know where to look for the right docs or know who to ask for where to look for the right docs (perldoc perlvar, in the case of Perl...I assume there's something similar for Perl 6, and I don't remember what it is for Python).