use IPC::Open2 qw( open2 );

my $pid = open2(my $to_child, my $from_child, 'konqueror', $url);
sleep(5);
kill(TERM => $pid);
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Since we don't care about pipes, the above boils down to

my $pid = fork();
if (!defined($pid)) {
   die("Unable to fork: $!\n");
}
if (!$pid) {
   exec('konqueror', $url);
   die("Unable to exec: $!\n");
}

sleep(5);
kill(TERM => $pid);
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Don't forget to cleanup zombies using waitpid.

Update: Added second snippet.
Update: Oops! I was using kill($pid, TERM);. Thanks, Fletch.