DB<159> use List::Util qw/sum/
 => 0

  DB<160> \@a
 => [0, 1, 0, 0, 1, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1]

  DB<161> $start=4;$stop=11
 => 11

  DB<162> $count{"a"}{$start}{$stop}=sum @a[$start..$stop]
 => 3