All of the XML parser modules expect raw bytes of XML as input. Therefore your results may differ if you parse from a file or open filehandle rather than from a string - it all depends on how the data got into the string.
If you pass a string of XML to XML::Parser you need to be sure that it is a byte string and not a character string. So the anonymous monk's suggestion to 'use utf8;' is exactly the wrong thing in this case - it would convert all non-ASCII literal strings in your script to Perl's internal character string representation. To convert from that to a form that an XML parser can read you'd need to use something like:
my $bytes = Encode::encode_utf8($string);
Perl's internal character string representation is similar to but not exactly the same as UTF8. In particular, some characters in the range U+0080 to U+00FF are represented as a single byte (the ISO8859-1 form) instead of the 2 bytes you'd expect from UTF8.