But since system only returns to your script when the executed command exits, you have to fork first, and in the child do an exec instead of a fork.

You might also find these threads interesting: execute secondary script not waiting for a result and continue with script and particularly Backticks and SIGALRM

(Update: added links; second update: make second link point to root node of the thread)

fork returns the child's PID in the parent process, and when the child process execs, the PID doesn't change.

This might get you what you want:

Update: Corrections thanks to ikegami.

...
my $pid = open CMD, "iceweasel -new-window '$url' |" or die $!;
...
kill 'TERM', $pid;

## Maybe also use waitpid here?
[download]

It actually creates a pid and kill it, but the window of iceweasel does not disappear. Which is strange because the pid is the one of iceweasel. my $url="http://www.google.fr/"; my $pid = open CMD, "iceweasel -new-window '$url' |" or die $!; print $pid; sleep 10; kill $pid; close CMD;

kill $pid;

You are missing an argument to kill. You need to specify the signal to send to the pid, either as a string , 'TERM' or 'KILL' or as a number: 2 or 9;

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my $url="http://www.google.fr/"; my $pid = open CMD, "iceweasel -new-window '$url' |" or die $!; print "$pid\n"; sleep 10; my $k=kill 'KILL',$pid; print "$k\n"; close CMD; the process is actually killed, but the window opened doesn't shut.