I'll just give you my personaly opinion...

The major change between 5.6.x and 5.8.x is the way Perl handles UTF8. The "IO layers". Perl 5.8.x will, by default, do an automatic translation between the internal character set (either ISO-Latin-1 as bytes, or UTF-8, commonly), and the character set your file handle is bound to — on most systems that is Latin-1. Perl 5.6.x didn't do that at all. And I'm not convinced whether I really like the behaviour, of either, the old and the new.

Anyway, recommended reading is, in short:

• binmode in perlfunc
• open in perlfunc
• PerlIO about IO layers
• Unicode I/O about automatic conversion between character sets

Anyway, that's the reason why you can get new warnings on Perl 5.8.x that you never had on Perl 5.6.x, something like "output of a wide character" (I should check the exact phrasing when I have access to a modern Perl, later today), when the target character set doesn't contain a translation for some Unicode character, and thus, the conversion fails.

update The exact phrasing of the warning is "Wide character in print". Well, I think I was pretty close.