Assuming the strings have the same length.

#!/usr/bin/perl

# https://perlmonks.org/?node_id=1218306

use strict;
use warnings;
use Algorithm::Diff qw(traverse_balanced);

compare( 'TTTATTT', 'TTTTTTT' );
compare( 'TTTTTTT', 'TBTTTTT' );

sub compare
  {
  my (@from, @to);
  print "compare '$_[0]' to '$_[1]', differences are";
  traverse_balanced( [ @from = split //, $_[0] ], [ @to = split //, $_
+[1] ],
    {
    CHANGE => sub { print " '$from[$_[0]]'\@$_[0]->'$to[$_[1]]'\@$_[1]
+" },
    } );
  print "\n";
  }
[download]

Outputs:

compare 'TTTATTT' to 'TTTTTTT', differences are 'A'@3->'T'@3
compare 'TTTTTTT' to 'TBTTTTT', differences are 'T'@1->'B'@1
[download]