#!/usr/bin/perl
my @seqs=qw(AAAA  ABAB CCC DDDDDDD EEEEFFF);
foreach my $seq (@seqs) {
    if($seq =~m/^(\w)\1{2,6}$/) {
        print "$seq: REPEATED\n";
    } else {
        print "$seq: NOT REPEATED\n";
    }
}

Outputs:
AAAA: REPEATED
ABAB: NOT REPEATED
CCC: REPEATED
DDDDDDD: REPEATED
EEEEFFF: NOT REPEATED