To check whether data are valid UTF-8 is rather straightforward. Here's the example, slightly modified from the synopsis of Encode: 
use Encode qw(decode encode);
$characters = decode('UTF-8', $octets, 
                     Encode::FB_CROAK | Encode::LEAVE_SRC);

[download]

This code will die if there are invalid data, so you would wrap it into the exception handler of your choice, plain eval and Try::Tiny seem to be popular.

BTW: as jcb already indicated, chances are excellent that if data pass as UTF-8, they actually are UTF-8. All bytes of multibyte characters in valid UTF-8 strings are in the range \x80 to \xFF, and in particular the bytes 2-4 are in the range \x80-\xBF. You just can't build readable text from characters in that range in any of the ISO-8859-* encodings, and about half of that range are "unprintable" control characters from ISO/IEC 6429.

In reply to Re^3: Perl detect utf8, iso-8859-1 encoding by haj
in thread Perl detect utf8, iso-8859-1 encoding by swiftlet

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.