There are several ways in Perl that you can run another process from within your Perl program:

1. You can use a system call, which runs the other process independently and returns to you when it has completed. You can check the completion status of the system call by examining $?. Before it runs, system flushes all Perl files open for output before it runs the process. (from 5.6.0)

Example:
  system("ls");


2. You can run a command in backtics (or using qx). In this case, the command runs to completion and the results are then returned to you as a string.

Example:
  $results = `ls`;

3. You can open a command (process) on a file handle, using a pipe character | as the "direction" character. And you can then read from, or write to, that file handle to pick up results as the command runs.

Example:
  open (FH,"ls|");
  while (<FH>) {
    print ;
    }

In the first two cases, the command runs to completion before your Perl program carries on; in the third case, the command is running in parallel to your Perl program and you can read from before it is completed - that's very useful for handling something like a monitoring program which you want to interactively summarise / report on.

There are examples of all three syntaxes [here] - written as a demonstration during last week's Perl Programming Course.
(written 2012-12-03, updated 2012-12-08)

 P223 - Perl - Interprocess Communication
  [604] Perl - multiprocess applications - (2006-02-13)
  [1918] Perl Socket Programming Examples - (2008-12-02)
  [2694] Multiple processes (forking) in Python - (2010-03-25)
  [2970] Perl - doing several things at the same time - (2010-09-25)
  [3010] Children, zombies, and reaping processes - (2010-10-23)
  [3011] What are .pid files? - (2010-10-23)
  [3412] Handling binary data in Perl is easy! - (2011-08-30)

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Building an object based on another object in Perl
Run other processes from within your Perl program
Lots of ways of doing the same thing in Perl - list iteration
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