How about a positive look ahead assertion?, since you only wanted to pick the string that contained the pattern provided, you don't have to capture the match and instead it would only be sufficient if you lookedaround for its existence. Reading the documentation for Regexes to walk you through can be a good idea ..

#!/usr/local/bin/perl
use strict;
use warnings;

#f = first, s = second.

my $f='ABCAAAABBBCCCCCAAABBBCCC';
my $s='ASRGRTGRT89579843rrrrrr';

print $f=~/(?=^[ABC]+$)/? "Matches [ABC] Combinations\n": "No pattern 
+found!\n";
print $s=~/(?=^[ABC]+$)/? "Matches [ABC] Combinations\n": "No pattern 
+found!\n";
[download]

#OUTPUT:
Matches [ABC] Combinations
No pattern found
[download]

Update: added a link that might benefit the OP.