You could try something like this:

#!/usr/bin/perl -l
# vi:ts=4 sw=4 et:
use strict;
use warnings;

my %hash = (
    apple   => 'red',
    ball    => 'red',
    lemon   => 'yellow',
    lime    => 'green',
    grass   => 'green',
);

my $search = 'green';

# get all keys, which have $search as value
my @keys = grep { $hash{$_} eq $search } keys %hash;

print "@keys";
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