Hmm, Do You think this is a perl bug?

I'll try to test new perl version.

Yes, it was a Perl bug. There were issues with how case insensitive matching worked within bracketed character class ranges. Here is the demo that shows it used to not work, and now does.

% head /tmp/t?
==> /tmp/t1 <==
use utf8;
print 'бБ' =~ /^[а-я]+$/i ? 'regexp ok' : 'regexp fail', "\n";

==> /tmp/t2 <==
use utf8;
print 'Бб' =~ /^[а-я]+$/i ? 'regexp ok' : 'regexp fail', "\n";

==> /tmp/t3 <==
use utf8;
print 'Бб' =~ /^[\x{430}-\x{44f}]+$/i ? 'regexp ok' : 'regexp fail', "\n";

% head /tmp/t? | uniquote
==> /tmp/t1 <==
use utf8;
print '\N{U+431}\N{U+411}' =~ /^[\N{U+430}-\N{U+44F}]+$/i ? 'regexp ok' : 'regexp fail', "\n";

==> /tmp/t2 <==
use utf8;
print '\N{U+411}\N{U+431}' =~ /^[\N{U+430}-\N{U+44F}]+$/i ? 'regexp ok' : 'regexp fail', "\n";

==> /tmp/t3 <==
use utf8;
print '\N{U+411}\N{U+431}' =~ /^[\x{430}-\x{44f}]+$/i ? 'regexp ok' : 'regexp fail', "\n";

% head /tmp/t? | uniquote -v
==> /tmp/t1 <==
use utf8;
print '\N{CYRILLIC SMALL LETTER BE}\N{CYRILLIC CAPITAL LETTER BE}' =~ /^[\N{CYRILLIC SMALL LETTER A}-\N{CYRILLIC SMALL LETTER YA}]+$/i ? 'regexp ok' : 'regexp fail', "\n";

==> /tmp/t2 <==
use utf8;
print '\N{CYRILLIC CAPITAL LETTER BE}\N{CYRILLIC SMALL LETTER BE}' =~ /^[\N{CYRILLIC SMALL LETTER A}-\N{CYRILLIC SMALL LETTER YA}]+$/i ? 'regexp ok' : 'regexp fail', "\n";

==> /tmp/t3 <==
use utf8;
print '\N{CYRILLIC CAPITAL LETTER BE}\N{CYRILLIC SMALL LETTER BE}' =~ /^[\x{430}-\x{44f}]+$/i ? 'regexp ok' : 'regexp fail', "\n";

% apply perl5.12.3 /tmp/t[123]
regexp ok
regexp fail
regexp ok

% apply perl5.14.0-RC1 /tmp/t[123]
regexp ok
regexp ok
regexp ok

The problem evaporates upon upgrading.

You can get the uniquote script demo’d above, plus several other (mostly) Unicode-oriented tools, from training.perl.com/scripts. Most are in varying states of pre-release-ness, but all of them do get used nearly daily.