#!/usr/local/bin/perl

use strict;
use warnings;

my @numbers = (1..2000000);
printf "Percentile %d%% at %f\n", $_, percentile($_,\@numbers)
for qw/25 75/;

sub percentile {
    my ($p,$aref) = @_;
    my $percentile = int($p * $#{$aref}/100);
    return (sort @$aref)[$percentile];
}