The differences are listed in the perldelta files between the versions. From 5.004 to 5.8, there have been many enhancements, from lexical filehandles to new features in regular expressions and unicode handling. From what I know, no functions have been deprecated.

But if you have no peculiar itch to scratch, why are you upgrading? If the (really old, I admit) version of Perl you have works for you, I would not upgrade.

My tip is to install 5.8.7 in parallel, say under /opt/perl/5.8.7, so that you have both versions of Perl available. I'm not aware of Sun utilities that rely on a specific version of Perl, but I would not risk losing them by upgrading the system perl interpreter.

Perl 5.8.7 should compile (and work) well under Solaris.