does X::Syntax
Syntax error thrown when some piece of code tries to use one of the old Perl variables (and it does not error for some other reason).
say $];
dies with
Unsupported use of $] variable; in Raku please use $*RAKU.version or $*RAKU.compiler.version
For every unsupported variable (which include most $^'letter' constructs, as well as others like $", the error message will mention that the variable is unsupported and the equivalent commands you could use.