If you install over the default location, or if the new location is found before the sun-installed perl in root's path, things _will_ break.

Of course, it's been ~4 years since I've used Solaris 8, and I can't remember exactly what it was that broke on us (I wasn't the one who encountered the problem) ... some admin utility, I think ...

/usr/local/bin/perl is safe, as is /opt/bin/perl, so long as you haven't adjusted root's path. Use whatever compiler you like, so long as you're consistent when compiling Perl modules.