my %uniq; 

my $s = "ABCABCABC1231234123123ABCABCABC123TestTestTestTestTest1234123123123121212121212121212121ppppp";;

print for grep{ ++$uniq{$_} == 1 } $s =~ m[(?=(.+)\1{4})]g;;
Test
1212
2121
12
21
p