#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my %lookup;
$lookup{'red'} = 77;
$lookup{'blue'} = 33;

sub get_by_value {
    my $val = shift;
    return [grep { $lookup{$_} == $val } keys %lookup];
}

die Dumper get_by_value(77);