Because you switched encoding and decoding to/from UTF-8: 
#!/usr/bin/perl
use strict;
use warnings;
use utf8 ;

use open IO => ':encoding(UTF-8)', ':std';

use Encode;
use Data::Dump qw/dd pp/;
use Devel::Peek;
use MIME::Base64 ;

my $str ='ř';

warn "\n*** orig str='$str'";
Dump $str;

my $bytes = Encode::encode_utf8($str);
warn "\n*** encode utf8='$bytes'";
Dump $bytes;

our $encoded = MIME::Base64::encode_base64($bytes);
warn "\n*** encode_b64 encoded='$encoded'";
Dump $encoded;


warn "\n*** decode_b64";
our $decoded = MIME::Base64::decode_base64($encoded);
Dump $decoded;

warn "\n*** encode_utf8";
$decoded = Encode::decode_utf8($decoded);
Dump $decoded;

warn "\n*** _utf8_on (NO-OP)";
Encode::_utf8_on($decoded);
Dump $decoded;

[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

[download]

In reply to Re: Converting utf-8 to base64 and back by choroba
in thread Converting utf-8 to base64 and back by LanX

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.