If the offending characters are not Unicode, just 8-bit, you could try use bytes;. It turns off Unicode processing so you can just process strings as a sequence of raw bytes. From the pod:

$x = chr(400);
print "Length is ", length $x, "\n";     # "Length is 1"
printf "Contents are %vd\n", $x;         # "Contents are 400"
{
   use bytes;
   print "Length is ", length $x, "\n"; # "Length is 2"
   printf "Contents are %vd\n", $x;     # "Contents are 198.144"
}
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