Adds exception for Perl 5 variables

Refs #2632

Author: JJ

doc/Type/X/Syntax/Perl5Var.pod6

@@ -0,0 +1,25 @@
+=begin pod :kind("Type") :subkind("class") :category("exception")
+
+=TITLE class X::Syntax::Perl5Var
+
+=SUBTITLE Compilation error due to use of Perl 5-only default variables
+
+    class X::Syntax::Perl5Var does X::Syntax { }
+
+Syntax error thrown when some piece of code tries to use one of the old Perl
+5 variables (and it does not error for some other reason).
+
+=for code :skip-test<Throws an exception>
+say $];
+
+dies with
+
+=for code :lang<text>
+Unsupported use of $] variable; in Perl 6 please use $*PERL.version or $*PERL.compiler.version
+
+For every unsupported variable (which include most C<$^'letter'> constructs,
+as well as others like C<$">, the error message will mention that the
+variable is unsupported and the equivalent commands you could use.
+
+
+=end pod