Add comment why seemingly dead code is necessary

src/pl/plperl/plperl.c

@@ -832,7 +832,15 @@ pp_require_safe(pTHX)
 		RETPUSHYES;
 
 	DIE(aTHX_ "Unable to load %s into plperl", name);
-	return NULL;				/* keep compiler quiet */
+	/*
+	 * In most Perl versions, DIE() expands to a return statement, so the next
+	 * line is not necessary.  But in versions between but not including 5.11.1
+	 * and 5.13.3 it does not, so the next line is necessary to avoid a
+	 * "control reaches end of non-void function" warning from gcc.  Other
+	 * compilers such as Solaris Studio will, however, issue a "statement not
+	 * reached" warning instead.
+	 */
+	return NULL;
 }