There are several ways to invoke a programm. One of the main differences is in the returning value.

• system("wc -l");
  Will call the given command and return the return value of that command. This is usually not what you want, because most of the times wc -l will mean that you want to get the number of lines back from that call and not if that call was successful or not.
• $nol = `wc -l`
  The backticks call the command and return it's output into the variable (here $nol. In this case this will be what you want.
• Another way of doing this is to use
  $nol = qx/wc -l/;
  (mnemonic: qx quote execute). I think is just the same as the backquotes (at least I don't know any difference)
• Of course there are other ways (exec,fork) that behave different with respect to processes, but I don't know much about this

Hope this helps...