Well fork is not only a perl command, but an unix command too.
What does fork do ?
It creates a copy of the actual running process. That's all!

What is this good for ?
Imagine you want to do several things at one time.
For example to count forward from 1 to 10 and at the same time backward from 10 to 1.
So you start like this:

for ($i=1;$i<=10;$i++) { print "$i\n" };
for ($j=10;$j>=1;$j--) { print "$j\n" };
# This will give you:
1
2
3
4
5
6
7
8
9
10
10
9
8
7
6
5
4
3
2
1
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OK this is done one after the other in one process

To do it at the same time you can use fork:

$pid = fork();
if (! $pid) {  ### This is the new process
   for ($j=10;$j>=0;$j--) { print "$j\n" };
} else {       ### This is the parent process
   for ($i=1;$i<=10;$i++) { print "$i\n" };
}
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What ??

Well with  $pid = fork() you create the same process again. This get's a new process-id and runs from the same point, where the fork was being done. The only difference between the parent and the child is, that only in the parent process the $pid variable is filled with the processnumber of the child process. So you can differ between the parent process ($pid=child-pid) and his child ($pid=empty).

Now you can do whatever you want in the if (! $pid) part.
It's important that all the vars declared and filled before the fork from the parent process are also available in the child process, but they are not visible in the other process!

Hope this makes it a bit more clearer, but I'm not sure :-)

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