pass example compilation test

Author: coke

doc/Language/variables.pod6

@@ -1255,11 +1255,13 @@ is very simple: define your own C<$/> inside the scope where you are
 doing the matching.  For example, taking a source of text, running
 a regex on it, and map that to a hash using parallel execution:
 
-    my %mapped = @source.race.map: {
-        my $/;  # need one in this block to prevent racing issues
-        m/foo (.*?) bar (.*)/;  # matches on $_, stores in $/
-        $0 => $1   # short for $/[0] / $/[1]
-    }
+=begin code :preamble<my @source>
+my %mapped = @source.race.map: {
+    my $/;  # need one in this block to prevent racing issues
+    m/foo (.*?) bar (.*)/;  # matches on $_, stores in $/
+    $0 => $1   # short for $/[0] / $/[1]
+}
+=end code
 
 X<|error variable>
 =head3 The C<$!> variable