You're right, you do need to sleep. Here's what I use in one of my programs to wait for a window:

sub WaitFor {
    my $win;
    my $title = shift;
    while (!defined $win) {
        select(undef,undef,undef,0.1); # sleep for 1/10th of a second
        ($win) = FindWindowLike(0, $title);
    }
    return $win;
}
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This sub takes the window title as a parameter and then just sleeps until that window appears. It doesn't set it to the active window, but you can do that with something like this:

SetForegroundWindow(WaitFor("Some Title"));
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Update: Remember to keep the parentheses around the $win in the loop, because FindWindowLike returns a list.