I think you can use pack and unpack to accomplish this:

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;
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If you are using a very recent version (i.e. experimental) of perl you can use the Encode module's decode_utf8 function, or use the internal function utf8::decode which it is implemented with:

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.
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(update: (Because it may seem odd:) The rational behind the decode in the function names is that these functions "[decode] from UTF-8 to a sequence of logical characters." )</o>

Also in the experimental version of perl the Encode module provides a _utf8_on function that will turn on that flag which says "this value carries UTF-8", however the documentation advises that this function is internal and may change, and that one must be very sure you are dealing with valid UTF-8 first.