my @dipept = qw( AABBCC AABBAA );
use v5.12;
use List::Util qw(sum);
my $count1  = sum map { scalar(my @r = /AA/g) } @dipept;
my $count2  = scalar map { /AA/g } @dipept;
my $count3  = 0;  $count3 += /AA/g for @dipept;
my $count5  = 0;  map { $count5 += /AA/g } @dipept;
my $count6  = 0;  grep { $count6 += /AA/g } @dipept;