So let's think this through.

1. "select a file from the command prompt"
   I'm not familiar with any form of "command prompt" -- familiarity being limited to RT-11, cpm, windoze & *nix -- that allows anyone to "select" anything. So I guess that means I'm not really going to be able to satisfy the "command prompt" part of your spec... or else that "command prompt" means something I don't understand... or else that "command prompt" means something other than what you appear to believe.
2. "user to be able to use commands like CD"
   Well, you could spawn a new CLI with backticks (neither exec nor system will work for what appears to be your purpose) and let the user play, but that's not really a terribly secure idea, even if you're writing a program for installation on the user's own machine. So, 'fergeddaboudit'!
3. "wanna stor the result in a string"
   Yeah, that would be really cool... but if you want to do that, you probably better build a webpage, a couple scripts to (1) deal with the form you're going to put in the html and (2) to do the work to achieve whatever your ultimate (and un-articulated) goal may be
   ...or
   step onto the learning curve for building a gui (It may be worthwhile to read perldoc Tk).

Bottom lines:

• Write a better spec.
• Read On asking for help, How do I post a question effectively? and, for good measure, Writeup Formatting Tips.
• Show us that you've tried something more than just posting a question that translates to 'Plz write me some free code.'
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