You can make the array index start in zero:

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

my $data = [];

my $start = 10000;
my $end = 30000;

$data->[$_ - $start] = rand for ($start .. $end);

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

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

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

    my $count = $end_point - $start_point + 1;
    my $amount;
    $amount += $data->[$_ - $start] for ($start_point .. $end_point);

    return $amount / $count;
}
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