Hmm, Do You think this is a perl bug?

Yes. And fixed somewhere between 5.12.2 and 5.14.0-RC1. Curious fact: 

binmode STDOUT, ':encoding(UTF-8)';
print 'Бб' =~ /^(а-я+)/i ? "regexp ok '$1'" : 'regexp fail', "\n";
# PS: perlmonks does't really do Unicode, it's your second example
# but without the $

Prints "regexp ok 'Бб'" - so it matched the whole string, and only the $ failed.

Perl 6 - second systems done right

In reply to Re^3: Regular Expressions, ignore case and unicode by moritz
in thread Regular Expressions, ignore case and unicode by OlegG

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