There is a command line utility named perldoc with which you can read perl documentation (and the documentation is also on the internet, if you prefer googeling. See http://perldoc.perl.org). If, for example, you do

perldoc -f exec
[download]

it conveniently jumps to where the function is explained. And the first sentence there will tell you that "exec executes a system command and never returns"

If you read the other two (and I'll concede that the backticks documentation isn't that easy to find with perldoc), you will find out that the biggest difference between system and the backticks is that the backticks return the output of the system command and system does not