can(METHOD)

    can checks to see if its object has a method called METHOD. If it does
    then a reference to the sub is returned; if it does not then undef is
    returned.

##</code><code>##

    package A;

    # stuff taken out

    sub get_write_file_func {
        my $self = shift;
        return $self->can("_write_file");
    }

##</code><code>##

    package A::B;
    use A;

    use vars qw/@ISA/;
    @ISA = qw/A/;

    # stuff taken out

    sub _write_file {
        my($path, $val, $start) = @_;
        # ...
    }