package Thing;

sub new {
    my($class, %option) = @_;
    my $self = {
        backend => "Thing::Backend::".$option{backend},
    };
    return bless $self, $class;
}

sub backend {
    my($self) = @_;
    return $self->{backend};
}

package Thing::Backend::A;

sub do {
    print "doing A\n";
}

package Thing::Backend::B;

sub do {
    print "doing B\n";
}

package Thing::Backend::C;

sub do {
    print "doing C\n";
}

package main;

my $x = Thing->new( backend => 'A' );

$x->backend->do();