This is the standard "pack of cards" algorithm that I mentioned in Re: Random data generation., misunderstanding BrowserUk's question. Changing the code for your case would give: 
use strict;
use warnings;

my $nRepeats = 1;
my @sSet = (4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20);
my $nLength = 8;

my @sAll;
for (1..$nRepeats) {
    push (@sAll, @sSet);
}

if ($nLength - 1 > $#sAll) {die "Don't be silly"}

my $sString;
for (1..$nLength) {
    my $i = int(rand(@sAll));
    $sString .= $sAll[$i];
    $sAll[$i] = $sAll[-1];
    pop(@sAll);
}

print "$sString \n";

[download]
I haven't tested this variation, but I tested the original.

Regards,

John Davies

In reply to Re: getting random number 8 times never the same by davies
in thread getting random number 8 times never the same by Anonymous Monk

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