my $utf8;
{ local $^W; # warnings are thrown when unpacking
             # invalid UTF8, ignore them, we'll check
             # for problems be seeing if the string is
             # null at the end and shouldn't be.
  $utf8 = pack "U*", unpack "U*", $raw;
}
deal_with_malformed_utf8()
  if length $utf8 == 0 and length $raw != 0;


##</code><code>##

use Encode;
my $string = decode_utf8($bytes);
deal_with_malformed_utf8() unless defined $string
# or
utf8::decode($string = $bytes)  # of course, you can just
  or deal_with_malformed_utf8();# use one variable, and not
                                # keep the raw bytes.