You've probably not installed the full perl distribution. If I'd had to guess, solaris splits up the perl distro into several packages, and you're on a system that only has the runtime part installed, but I don't have a solaris machine here to check.

You might want to search for and install something like perl-dev or perl-devel (i.e. the perl development package).

Also, depending on circumstances I would suggest you get a more recent version of perl. 5.6.1 is 6 years old by now, and if you want to do anything with unicode (or possibly threads) you need at least 5.8.0 but preferably 5.8.8 (the current stable version). Check http://www.sunfreeware.com/