use warnings;
use strict;

my $length = 2;
my $str = "BEBEBEHUHUHUJJFAFALL";

my %cnt;
while ($str =~ /(.{$length})/g) {
    $cnt{$1}++;
}

print "Found:\n";
print "$_ $cnt{$_}\n" for sort keys %cnt;

__END__

Found:
BE 3
FA 2
HU 3
JJ 1
LL 1