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http://validator.w3.org/check?uri=http%3A%2F%2Fwww.websitedev.de%2Fmarkup% 2Fvalidator%2Ftests%2Fdouble-utf-8-bom.html Is invalid, http://validator.w3.org/check?uri=http%3A%2F%2Fvalidator.w3.org%2Fcheck%3Furi% 3Dhttp%253A%252F%252Fwww.websitedev.de%252Fmarkup%252Fvalidator%252Ftests% 252Fdouble-utf-8-bom.html Sorry, I am unable to validate this document because on line 172 it contained one or more bytes that I cannot interpret as utf-8 (in other words, the bytes found are not valid values in the specified Character Encoding). Please check both the content of the file and the character encoding indication.
This is probably a duplicate of the bug that deals with using the UTF-8 flag for truncate_line() etc.
Offtopic, but BOM related: this might be interesting sometime: http://search.cpan.org/dist/File-BOM/
One way to deal with this is to pass our complete output data through the UTF-8 checker (charlint), possibly modified to tag illegal byte sequences and continue instead of croaking. BTW, cf. Comment #1, I can't seem to find this bug you're refering to; care to provide a bug number?
See the relevant comment in the source about Perl 5.8.x --- and I am not sure how your suggestion would help. The problem here is that the string is considered a byte string and thus substr etc. do not work as expected.
Fixed in HEAD.