sub FETCH {
    my ($this, $key) = @_;
    return $this->{l}{$key} if exists $this->{l}{$key};

    # I don't know what you're doing, but eval is scary :)
    # Consider using a sub ref (sub { ... } or \&subname) instead
    $this->{l}{$key} = eval $this{f}{$key}; 

    # Create a new hash ref if the eval returned undef
    $this->{l}{$key} = { } if not defined $this->{l}{$key};

    return $this->{l}{$key};
}