I am still not sure what your input is, but I hope you might find one of the following two solutions helpful: 
use Data::Dumper;

my $string = 'MHDLKNDHASDRWT';
my %count_string;
$count_string{$_}++ for $string =~ /(?=(..))/g; # Uses look-ahead to o
+nly progress by one character.

my @array = split //, $string;
my %count_array;
$count_array{ join q(), @array[$_, $_ + 1] }++ for 0 .. $#array - 1;

print Dumper \%count_string, \%count_array;

[download]
Note: /AA/ matches the capital letter A followed by the capital letter A. It does not stand for "anything" in regular expressions.

Updated: Added the hashes.

لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re^3: Using grep in a scalar context by choroba
in thread Using grep in a scalar context by newbie1991

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