#!/usr/bin/perl

package ValideHashValue;

use Tie::Hash;
use base 'Tie::StdHash';
use strict;
use warnings;

sub TIEHASH{
    my $class = shift;
    my $href  = shift;
    return bless $href, $class;
}

sub FETCH{
    my $self = shift;
    my $key  = shift;
    return defined $self->{$key} ? $self->{$key} : 'das war mal undef';
}

package main;

use strict;
use warnings;

my %hash = ();
print $hash{foo}; # Use of uninitialized value in print ...

# now, using tie:
tie my %h, 'ValideHashValue', \%hash;

print $h{foo}; # das war mal undef