Apparent bug in Perl 5.10 regexes w. UTF-8 expression

I've found a place where Perl seems to behave differently depending on whether something is marked as UTF-8 or not, regardless of the fact that it is just ASCII. In the following code snippet, use strict; use Encode 'decode'; use Lingua::JA::FindDates 'subsjdate'; binmode STDERR,"utf8"; binmode STDOUT,"utf8";