This command in bash works fine (Perl 5.30 shipped with Cygwin):

git-md-toc -ut "some-text-in-russian" -Tutf8 "utf8-cyrillic.md"

But it fails in DOS sessions -- the title is being added in wrong encoding. To resolve the issue I have to use one more option (standalone StrawberryPerl 5.30):

##</code><code>##

#!/usr/bin/perl --
use strict;
use warnings;
use utf8;

system
    $^X,
    q{-e},
    q{
use strict;
use warnings;
use Encode;
use Data::Dump;
my @argv = map{  decode('UTF-8', $_,     Encode::FB_CROAK)  } @ARGV;
dd\@argv;

    }, 
    qq{\x{43A}\x{435}\x{431}\x{430}\x{431}}, # kebab
# copy from html below cause perlmonks doesnt utf, so browser converts html entities 
    '&#1082;&#1077;&#1073;&#1072;&#1073;',
    ;

##</code><code>##

[
  "\x{43A}\x{435}\x{431}\x{430}\x{431}",
  "\x{43A}\x{435}\x{431}\x{430}\x{431}",
]