dynamically add leading underscore to existing sub

Question

In this example for perl book, where is "_components" coming from? I know adding _ to indicate sub is private to the package but this one seems to use leading score out of nowhere. Can you please point me to some source about this? thank you.

package Computer;
@ISA = qw(StoreItem);

sub new {
    my $pkg = shift;
    my $obj = $pkg->SUPER::new("Computer", 0,0);
    $obj->{_components} = [];
    $obj->components(@_);
    $obj;
}

sub components {
    my $obj = shift;
    @_ ? push (@{$ojb->{_components}}, @_) : @{$obj->{_components}};
}

sub price {
    my $obj = shift;
    my $price = 0;
    my $component;
    for my $component ($obj->components()) {
        $price += $component->price();
    }
    $price;
}

Answer 1

_components is a literal key for the $obj hash reference. It is not "coming from" anywhere. This is a quirk of Perl syntax. In the statement

$obj->{_components} = [];

$obj is a reference to a newly created instance of a class (in the prior statement) and _components is being defined as a key in that instance, and being initialized to a reference to an empty array. This is equivalent to

$obj->{'_components'} = [];

For instance

$ perl -de0

Loading DB routines from perl5db.pl version 1.55
Editor support available.

Enter h or 'h h' for help, or 'man perldebug' for more help.

main::(-e:1):   0
  DB<1> $obj->{a} = "hello";

  DB<2> x $obj
0  HASH(0x800756bf8)
   'a' => 'hello'
  DB<3> p $obj->{'a'}
hello
  DB<4> p $obj->{a}
hello
  DB<5>