my $standard='ABCAAAABBBCCCCCAAABBBCCC';

my %seen;
my @unique_letters = grep{$seen{$_}++ <1 } split(//,$standard);
my $unique_letters = join("",@unique_letters);

# these above two lines could be combined, but I think it
# reads better this way, and YES, it is completely legal in Perl
# to have an array variable named unique_letters and a
# string named the same thing. Update: with same name not same "thing".

foreach (@strings)
{
   print "$_\n" if (/[^$unique_letters]/);
}
#prints:
#ASRGRTGRT89579843rrrrrr
#A98797BqrtoiquyrtoCafdgagfd
#change to "not" of these: (!/[^$unique_letters]/);
#to get: ABCAAAABBBCCCCCAAABBBCCC