I probably have a biased perspective, but I think the most dramatic differences in 5.8 relative to 5.005 involve the use of utf8 as the internal string representation, and all the amazing regex magic that this enables, plus the extensive encoding/decoding support for dealing with non-unicode character sets -- and (fireworks, please) Perl::IO.

Look at the perluniintro, perlunicode, Encode and Perl::IO docs, and notice how a lot of the perlfunc and perlre content has been enhanced to take these new facilities into account (esp. "open" and "binmode").

(I made the same leap -- also coming mostly from a solaris background -- from 5.5 to 5.8, so I don't really know what I missed in 5.6, or how much many of 5.8's extras were previewed there. That doesn't bother me; 5.8 is all I need now.)