Speed up bashrc startup

On my system (2GHz dual-core i7, SSD, MacOS 10.7) these patches more than halve the time it takes to source `~/perl5/perlbrew/etc/bashrc` (from ~0.18 sec down to ~0.08s). Other stuff in my `~/.bashrc` takes about 0.07s, so these patches reduce the delay for opening a new terminal window from ~0.25s (feels laggy; window takes a moment to come up) to ~0.15s (not quite instant, but feels much quicker).

And this is still a fairly hot machine, so I assume most users will benefit more than I do.

(Attacking the remainder of the startup time requires making App::perlbrew load and run quicker, which really means making local::lib load faster, which means that part is not so easy.)