# No two repeated characters

my @set = qw( a b c d e f );
my $len = 12;

my $s = '';
my $i = int(rand(@set));
for (1..$len) {
   $s .= $set[$i];
   $i = ( $i + int(rand($#set)) + 1 ) % @set;
}

print("$s\n");