Given that the exploit relies on using byte sequences that cannot be interpreted as valid utf8 strings, I would think that anyone writing a script that uses the "-T" flag, and expects to handle utf8 data from a tainted source, would prefer to read such input as ":raw", and always use Encode::decode() to convert it to perl-internal utf8 form.

And in doing so, it would usually be prudent to do it like this (adapting the sample code given in the OP): 

#!/usr/bin/perl -T

use strict;
use Encode;

%ENV = (
    PATH => '/usr/bin'
);

open my $filehandle, "< :raw", "test.bin" or die $!;
my $word = readline $filehandle;

eval { $word = decode( "utf8", $word, Encode::FB_CROAK ) };

if ( $@ ) {
    warn "unusable input from test.bin\n";
}
else {
    my ($untainted) = $word =~ /^(\w+)$/;

    if ($untainted) {
    # It passed the regex, so it is "safe".
    system "echo $untainted";
    }
}

[download]

In reply to Re: UTF8 related proof of concept exploit released at T-DOSE by graff
in thread UTF8 related proof of concept exploit released at T-DOSE by Juerd

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.