Use a tied hash.

package DefaultHash;

sub TIEHASH {
    my $class = shift;
    my $default = shift;

    my $self;
    $self->{_DEFAULT} = $default;
    $self->{_HASH} = {};

    return bless $self, $class;
}

sub FETCH {
    my $self = shift;
    my $key  = shift;

    return 
        exists  $self->{_HASH}{$key}
        ?  $self->{_HASH}{$key}
        :  $self->{_DEFAULT}
}

sub EXISTS {
    my $self = shift;
    my $key  = shift;

    return exists $self->{_HASH}{$key};
}

sub FIRSTKEY {
    my $self = shift;

    my $foo = keys %{$self->{_HASH}};  # Reset each
    return each %{$self->{_HASH}};
}

sub NEXTKEY {
    my $self = shift;
    return each %{$self->{_HASH}};
}

sub CLEAR {
    my $self = shift;

    $self->{_HASH} = {};
}

sub DELETE {
    my $self = shift;
    my $key  = shift;

    return delete $self->{_HASH}{$key};
}

sub STORE {
    my $self = shift;
    my $key  = shift;
    my $value = shift;

    $self->{_HASH}{$key} = $value;
}

package main;

tie( my %hash, 'DefaultHash', 'a default value' );

%hash = ( a => 1, b => 2, c => 3 );

print "$_ => $hash{$_}\n" for qw(a b c d);
[download]