%hash = (
  100 => '1',
  101 => '23',
  102 => '4',
  ...
  200 => '75'
);

##</code><code>##

#!/usr/bin/perl
use strict;
use warnings;
use Benchmark qw(cmpthese);

my $data = {};

$data->{$_} = rand for (10000 .. 30000);

print average($data, 15000, 25000), "\n";

cmpthese(50, { orig => sub { average($data, 15000, 25000) } });

sub average {
    my ($data, $start_point, $end_point) = @_;

    my @keys = sort keys %{ $data };

    my $start_pos;
    my $end_pos;
    for (0 .. $#keys) {
        $start_pos = $_ if $keys[$_] eq $start_point;
        $end_pos   = $_ if $keys[$_] eq $end_point;
    }
    return undef unless defined $start_pos and defined $end_pos;

    my $count = $end_pos - $start_pos + 1;
    my $amount;
    $amount += $data->{$keys[$_]} for ($start_pos .. $end_pos);

    return $amount / $count;
}