This should do it

use strict;

my $cc    = "ABCDEF";
my @false = qw(ABDA ABXD AABB);
my @true  = qw(ABC A DEFA);

for(@false,@true) {
    my $res = /[^$cc] | ([$cc]) [$cc]* \1/x ? "no" : "yes";
    print "$_: $res\n";
}

__output__

ABDA: no
ABXD: no
AABB: no
ABC: yes
A: yes
DEFA: yes
[download]

HTH

_________
broquaint