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).
Unsupported use of $] variable; in Raku please use .version or .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.