How do I determine what my default shell is? If I'm root, how do I determine what my user's default shell is?

I think I'm running bash by default.

How do I write a perl program that can inject history into the shell's history so it can be retrieved with the history | grep command?

I have an old fashioned console (non-GUI) style perl program that asks dozens of questions. Incorporating readline might mitigate this a bit. What I would like to do is enhance the logic of this program to check ARGV before prompting the console (user) for information and then, after all the questions have been answered, artificially inject the completed command line into the shell history so the command can be re-run again with identical inputs by just hitting the up-arrow. This will save them from having to type all that stuff in again.

Can this be done?
Thanks!
Siegfried