This is just from memory, but I believe you'll want to set both make_arg and make_install_arg to:

-I/usr/home/broadsof/site_perl/lib
[download]

So, something like this might work:

% perl -MCPAN -e shell
> o conf makepl_arg PREFIX=/usr/home/broadsof/site_perl LIB=/usr/home/
+broadsof/site_perl/lib
> o conf make_arg -I/usr/home/broadsof/site_perl/lib
> o conf make_install_arg -I/usr/home/broadsof/site_perl/lib
> install Compress::Zlib
[download]

-Blake