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);
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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.