Hi, haj, ikegami and choroba, Corion, monks who have replied to this thread.
Haj, ikegami, it seems below code worked for me, so regex seems working for me. Thank you. 
 my $utf8_decodable_regex =
    qr/[\xC0-\xDF][\x80-\xBF]    | # 2 bytes unicode char
       [\xE0-\xEF][\x80-\xBF]{2} | # 3 bytes unicode char
       [\xF0-\xFF][\x80-\xBF]{3}/x;

    $testStr = decode('utf-8',$testStr);
    $testStr =~ s/($utf8_decodable_regex)/decode('utf-8',$1)/gex;
    $testStr = encode('utf-8',$testStr);

[download]
Though it's working, it would be great if you can explain why its working.
Cheers !!!

In reply to Re: How to avoid decoding string to utf-8. by Anonymous Monk
in thread How to avoid decoding string to utf-8. by Anonymous Monk

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.