Add test and docs about multiple identical block labels

Fixes #18369.

Author: khwilliamson

pod/perlsyn.pod

@@ -330,7 +330,9 @@ If the LABEL is omitted, the loop control statement
 refers to the innermost enclosing loop.  This may include dynamically
 searching through your call-stack at run time to find the LABEL.  Such
 desperate behavior triggers a warning if you use the C<use warnings>
-pragma or the B<-w> flag.
+pragma or the B<-w> flag.  If more than one label with the same name
+occurs, any reference to that name refers to the one labelling the
+innermost enclosing loop.
 
 If the condition expression of a C<while> statement is based
 on any of a group of iterative expression types then it gets

t/op/loopctl.t

@@ -36,7 +36,7 @@ BEGIN {
     set_up_inc(qw(. ../lib));
 }
 
-plan( tests => 67 );
+plan( tests => 69 );
 
 my $ok;
 
@@ -922,6 +922,23 @@ TEST41: {
 }
 cmp_ok($ok,'==',1,'dynamically scoped');
 
+TEST42: {   # GH #18369
+    my $outer = 0;
+    my $inner = 0;
+  L:
+    for my $i (0 .. 0) {
+        $outer++;
+
+      L:
+        for my $j (0 .. 0) {
+            $inner++;
+            redo L if $inner < 10;
+        }
+    }
+
+    is ($inner, 10, "redo label refers to innermost enclosing one");
+    is ($outer, 1,  "redo label doesn't refer to outermost enclosing one");
+}
 
 # [perl #27206] Memory leak in continue loop
 # Ensure that the temporary object is freed each time round the loop,