I think you're not following OOP too close. I would change it to something like this (untested):

sub new {
    my ($class, %parameters) = @_; 
    my $self = {};

    # here we check required parameters, to croak() them. since we are
+ using 
    # OOP, we *need* to ensure this object will be complete to perform
+ whatever
    # operations, avoiding coupling.
    foreach (qw(cardnum)) { # maybe we have other required parameters
        croak "Required parameter '$_' not found"
            unless exists $parameters{$_};
    }
 
    # now we set defaults for not obligatory parameters
    $self->{'tail'}        = $parameters{'tail'} || -4;
    $self->{'head'}        = $parameters{'head'} || 0;
    $self->{'replacement'} = $parameters{'replacement'} || "*";
}

sub obscure {
    my ($self) = @_;
    my $obscured = substr($self->{'cardnum'},
                          $self->{'head'},
                          $self->{'tail'}
    ) =~ s/./$self->{'replacement'}/g;

    return $obscured;
}
[download]

Hope this helps.