I've run libnet 1.0703 on Solaris 5.6 and perl 5.05003 for a couple of years with no problems, but I wouldn't worry about using a more recent version. As to installation, it's just perl Makefile.pl, make test, make install.
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Comment on Re: libnet (Net::FTP) for SPARC-Solaris 5.8
I've also run libnet 1.0703 against Solaris 8 for SPARC against perl 5.005_03.
Installation was simple as petral states and without problem. The latest version I have used recently is libnet 1.0901 and this installed without hitch against perl 5.6.1 on Solaris 8.