use strict;
use warnings 'all';

package A;

sub _write_file {
        print "Writing in package A\n";
}

sub write_file {
        my $self = shift;
        $self->_write_file(@_);
}

package A::B;
use base qw/A/;

sub new {
        my $class = shift;
        bless {} => $class;
}

sub _write_file {
        print "Writing in package A::B\n";
}

my $writer = A::B->new;
$writer->write_file;