Do you mean:
Any time you run a command on a Unix platform, system creates a fork, spawns a new sh, runs the command through exec, then exits the child shell. The same process also takes place when you run a perl script.

"the whole environment will be different. that is the reason the Band will not take any effect as ikegami said."

I'm not quite sure what you mean by these last two clauses though... Do you mean that perl defaults to the 'sh' shell when running system commands, and that this may differ from your personal settings?
In any case, thanks for the input!~
Joe