Try using backticks and you can bg it like you would any unix process from the command line, I am on WinXP and would probably use Win32::Process... but you can just do something like:
Tk's main loop keeps running until you exit... There is a better way to do non-blocking system calls however... you should do some reading of the perl/tk faq's or read about it in Mastering Perl/Tk which is an O'Reilly book ( ISBN 1-56592-716-8) I believe the Schwartz book you are referring to is "Learning Perl". The Perl/Tk book is all about the Perl/Tk implementation and assumes you already are familiar with Perl.