foreach my $sequence (@bigram) {
    #my $count;   is this a bug?
    my @word = split ' ', $sequence;
    my $prev = shift @word;
    foreach $word (@word) {
        $count{"${prev}_${word}"}++;
        $prev = $word;
    }
}

##</code><code>##

a b n z
d f n z
v

##</code><code>##

n_z occurs 2 times
a_b occurs 1 times
d_f occurs 1 times
f_n occurs 1 times
b_n occurs 1 times