I am sorry, I'v made a mistake:

my $n = 1000;
my $cnt = 10;
my @ret;
for (my $i = 0; $i < $cnt; $i++) {
    my $r = int(rand()*$n);
    my $j = 0;
    while ($j <= $r) {
        $r++ if defined $ret[$j];
        $j++;
    }
    $ret[$r] = 1;
    $n--;
}
    
print "$cnt random indexes:\n";
for (my $i = 0; $i < $n+$cnt; $i++) {
   print "$i\n" if $ret[$i];
}
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As you can see, the repaired algorithm does ~ $n*$cnt cycles. It is too naive. Therefore, it should be better to use some previously adviced libraries.