Considering that 10 is much smaller than 600, I'd do: 
my @array = ('-') x 600;
foreach (1 .. 10) {
    my $i = int rand 600;
    redo if $array[$i] eq 'X';
    $array[$i] = 'X';
}
my $joined_string = join "", @array;

[download]
In theory, this may run "for ever", but in practice it will run fast enough.

In reply to Re: Difficulty in randomization by JavaFan
in thread Difficulty in randomization by Anonymous Monk

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