Make bashrc use PERLBREW_ROOT at installation time

If the generated bashrc file includes (by default) the value of `PERLBREW_ROOT` at install time, the user doesn't have to remember to add a line in bashrc (or wherever) to also set PERLBREW_ROOT again. 

(This explanation's a bit convoluted, sorry...)

This might resolve some potential confusion for people (like me) who change `PERLBREW_ROOT` before installation, but then don't read the man page before adding the necessary `source` line to `~/.bashrc`.  In a new shell, `PERLBREW_ROOT` won't be set and so will default to `$HOME/perl5/perlbrew`, which won't exist. 

Because the bashrc file then uses the value of this variable to find the `perlbrew` *command*, later (when `__perlbrew_set_path` is called), the line that sets `PATH_WITHOUT_PERLBREW` will return nothing (since `perlbrew` isn't found). Consequently, sourcing perlbrew's bashrc has the net effect of setting the path to contain *only* the directory `$HOME/perl5/perlbrew/bin`, and clobbering anything else!

At any rate, having the generated bashrc inherit a default value of `PERLBREW_ROOT` from whatever it was at installation will avoid this problem for people like me (certainly I'm not alone) who don't read the full man page before firing up a new shell and running. (Alternately, you could just add a notice about export `PERLBREW_ROOT` before the notice about sourcing perlbrew's bashrc.)