use strict;
use warnings;

my $nRepeats = 2;
my @sSet = qw (a b c d e f);
my $nLength = 12;

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";