#!/usr/bin/env perl -l

use strict;
use warnings;

use List::Util qw{max};

my %data = (
    -4 => 0, -3.9 => 0, -3.8 => 0, -3.7 => 1, -3.6 => 0, -3.5 => 1, -3.4 => 0,
    -3.3 => 0, -3.2 => 5, -3.1 => 19, -3 => 23, -2.9 => 50, -2.8 => 43,
    -2.7 => 28, -2.6 => 72, -2.5 => 165, -2.4 => 302, -2.3 => 168, -2.2 => 260,
    -2.1 => 151, -2 => 13, -1.9 => 1, -1.8 => 0, -1.7 => 0, -1.6 => 0,
    -1.5 => 0, -1.4 => 0, -1.3 => 0, -1.2 => 0, -1.1 => 0, -1 => 0,
);

my $max_value = max values %data;
my $max_key = { reverse %data }->{$max_value};
my $half = $max_value / 2;
my @closest;
my $side = 0;

for my $key (sort { $a <=> $b } keys %data) {
    if ($key == $max_key) {
        $closest[$side] = [$max_key, $max_value] unless defined $closest[$side];
        $side = 1;
        next;
    }   

    my $diff = abs $half - $data{$key};

    if (! defined $closest[$side]) {
        $closest[$side] = [$key, $diff];
        next;
    }   

    $closest[$side] = [$key, $diff] if $closest[$side]->[1] > $diff;
}   

print "max_key = $max_key; max_value = $max_value";
print "half = $half";
print "closest: BEFORE = $closest[0]->[0] AFTER = $closest[1]->[0]";

##</code><code>##

max_key = -2.4; max_value = 302
half = 151
closest: BEFORE = -2.5 AFTER = -2.1