#!/usr/bin/perl

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

use strict;
use warnings;

my $a = 'TTTATTT';
my $b = 'TTTTTTT';
my $c = 'TBTTTTT';

print "compare $a to $b, difference is ",
  $a =~ s/[$b]+//gr, $b =~ s/[$a]+//gr, "\n";

print "compare $b to $c, difference is ",
  $b =~ s/[$c]+//gr, $c =~ s/[$b]+//gr, "\n";
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Outputs:

compare TTTATTT to TTTTTTT, difference is A
compare TTTTTTT to TBTTTTT, difference is B
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