#!/usr/bin/perl -w

use strict;

my %hash = ('a' => {
                      'value' => 1,
                      'foo' => 'bar',
                   },
            'b' => {
                      'value' => 2,
                      'foo' => 'bar',
                   },
            'c' => {
                      'value' => 3,
                      'foo' => 'bar',
                   },
           );

if (! $hash{'d'}->{'value'}) {
        print "That key doesn't exist. Valid keys are: ";
        print join (", ", keys %hash);
}