Net::Telnet lives in /usr/lib/perl5/site_perl/5.005/Net/Telnet.pm, on my Linux box. It should probably go in a similar place on yours (/usr/local/lib...).

It's probably more useful to ask how you installed it... did you use the CPAN module or download a tarball and run 'perl Makefile.PL; make; make test; make install'? Either way, it should install automagically to the right place. If not, that's a bug and needs to be reported.

Adding a directory to @INC can be found in How do I add a directory to my include path at runtime?, but it's probably better that the install works correctly.