X::Placeholder::Mailine. Explain "mainline" in the glossary

lib/X/Placeholder/Mainline.pod

@@ -0,0 +1,18 @@
+=begin pod
+
+=TITLE X::Placeholder::Mainline
+
+    class X::Placeholder::Mainline is X::Placeholder::Block
+
+Thrown when a placeholder variable is used in the mainline, i.e. outside of
+any explicit block.
+
+Example:
+
+    $^x
+
+Produces the error message
+
+    Cannot use placeholder parameter $^x in the mainline
+
+=end pod

lib/glossary.pod

@@ -109,6 +109,15 @@ Examples of things that are not lvalues
     sub f { }; f();          # "normal" subs are not writable
     sub f($x) { $x = 3 };    # error - parameters are read-only by default
 
+=head1 Mainline
+
+The C<mainline> is the program text that is not part of any kind of block.
+
+    use v6;     # mainline
+    sub f {
+                # not in mainline, in sub f
+    }
+    f();        # in mainline again
 
 =head1 Slurpy