+=head1 PERFORMANCE
+
+This module can provoke sizeable inefficiencies for regular expressions,
+due to unfortunate implementation details. If performance matters in
+your application and you don't need $PREMATCH, $MATCH, or $POSTMATCH,
+try doing
+
+ use English qw( -no_match_vars ) ;
+
+. B<It is especially important to do this in modules to avoid penalizing
+all applications which use them.>
+