#!perl -slw
use strict;
use List::Util qw/ shuffle /;


sub x {
    my( $N, @M ) = @_;
    my ($minus_1, $minus_2, $str) = (('') x 3);

    $minus_1 = $M[rand @M];
    $minus_2 = $M[rand @M];

    $str = $minus_1 . $minus_2;

    for (1 .. $N - 2) {
        my $next_char;
        my $idx = int(rand @M);
        if ($minus_1 eq $minus_2) {
            $idx = ($idx + int(rand @M) + 1) % @M;
        }
        $str .= $next_char = $M[$idx];
        ($minus_1, $minus_2) = ($minus_2, $next_char);
    }
    return $str;
}

for( 1 .. 10 ) {
    my $x = x( 12, qw[ A B C D E F ] );
    print $x, $x =~ m[(.)\1\1] ? ' Fail' : ' Ok';
}

__END__
[17:39:11.61] c:\test>junk3
BAADDBBCADEF Ok
ADAEFADBCEEA Ok
CECBAABFCEFE Ok
CDDCFDAABEBF Ok
ADFBFCEADFFF Fail
DAABAAFDCCCE Fail
ABAADCBDFBBB Fail
FDDDBEEADBCC Fail
CEDCDDFCEEFA Ok
FCBECCCEAFEB Fail