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";