When I first looked at this, I came up with something similar to what Cristoforo did.

#!/usr/bin/perl -w
use strict;

my @strings = ('ABCAAAABBBCCCCCAAABBBCCC',
               'ASRGRTGRT89579843rrrrrr',
               'A98797BqrtoiquyrtoCafdgagfd');                        
+    
foreach (@strings)
{
   # Here the scalar count value of tr is used 
   # tr is very fast but lacks the flexibility of
   # regex
   # prints the string unless it has some char that is
   # not an A, B, C.
   
   print "$_\n" if !(tr/ABC//c);
}
# Prints: ABCAAAABBBCCCCCAAABBBCCC
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This is a great idea if you know that ABC are the things in advance.

If we have a "standard" string by which others will be compared, and that string is a variable. We we are going compare that "standard" string against many lines, and create a string with the unique chars in the "standard" string and use that string in a simple regex to see if they are there. No fancy look ahead required.

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
[download]