According to 'perldoc lib':

use lib LIST;

is almost the same as saying:

BEGIN { unshift(@INC, LIST) }

The only difference is that, for each directory in the list, it checks for an architecture specific directory below. So, you've definitely hit on the right idea, but I think
use lib '/home/my_home_path/.cpan/CPAN'; is a little cleaner.

The other thing that might help is that you can configure CPAN to automate the exact sequence you describe. Run perl -MCPAN -e shell and do the following:

o conf makepl_arg 'PREFIX=your_path_here INSTALLDIRS=site INSTALLSITELIB=your_path_here' (That's a lowercase 'o', not a bullet point)
o conf on its own will show you your current conf(iguration) options.
makepl_arg defines the options that will be passed after perl Makefile.PL

Then, in the interactive shell, install File::Spec would be equivalent to the commands you listed above.