Just use the Encode module that comes with Perl. It does the RFC 2047 decoding in one step:

use Encode;

my $russkiy = decode("mime-header", "=?UTF-8?B?0KDRg9GB0YHQutC40Lk=?="
+);
[download]

$russkiy is a proper 7 character Perl unicode string.

PS- You can use Russian variable names too, if you wish:

use utf8;
use Encode;

my $русский = ...;

PPS- Perl Monks sucks for not allowing Unicode characters in <code>-blocks.