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

sub x {
    my( $N, @M ) = @_;
    my ($minus_1, $minus_2) = ($M[rand @M], $M[rand @M]);
    my $str = $minus_1 . $minus_2;

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

for( 1 .. 100 ) {
    my $x = x( 12, qw[ A B C D E F ] );
    print $x, $x =~ m[(.)\1\1] ? ' Fail' : ' Ok';
}