use warnings;
use strict;

while (<DATA>) {
  chomp;
  print "$_ -> ";

  my $len = length;

  if ( s/(\w)\1{2,}\Z/$1$1/ ) {
    $len -= length;
    print "$len -> $_";
  }

  print "\n"; 
}

__DATA__
ACTGCTAGGGGGGG
TCAGCTAGCNA
ACTGSCGACAAAA
GTCTGAGTTATTT