sub foo {
    my $self = shift;

    if (ref $self) {
        # Do stuff
        return $some_useful_value;
    } else {
        # Complain
        return undef;
    }
}

##</code><code>##

sub foo {
    my $self = shift;

    unless (ref $self) {
        # Complain
        return undef
    }

    # Do stuff
    return $some_useful_value
}