in reply to Favourite One-liners?
% perl -MCPAN -e shell [download]
Update: Thanks, BDF. Tidbits like this are why I hang around PM.
Perl comes with a "cpan" script that does that for you so you don't have to do all that typing. You can also use it just to install modules quickly:
cpan Foo::Bar Baz Quuz::Frob [download]