sub _init in Perl explanation

Question

I was wondering what this subroutine does in Perl. I believe i have the general idea but i'm wondering about some of the syntax.

sub _init 
{
  my $self = shift;
  if (@_) {
    my %extra = @_;
    @$self{keys %extra} = values %extra;
  }
}

Here is what i think it does: essentially add any "extra" key-value pairs to the nameless hash refereced by the variable $self. Also i'm not 100% sure about this but i think my $self = shift is actually referring to the variable $self that called the _init() subroutine.

My specific questions are:

1. Is $self actually referring to the variable that called the subroutine _init() ?
2. What does the @$ syntax mean when writing @$self{keys %extra} = values %extra;

Answer 1

Your understanding is correct. This allows the user of a class to set any parameters they want into the object.

1. Yes. If you call, for example, $myobject->_init('color', 'green'), then this code will set $myobject->{'color'} = 'green'.

2. This is a somewhat confusing hash operation. keys %extra is a list (obviously of keys). We are effectively using a "hash slice" here. Think of it like an array slice, where you can call @$arrayref[1, 3, 4]. We use the @ sign here because we're talking about a list - it's the list of values corresponding to the list of keys keys %extra in the hash referenced by $self.

Another way to write this would be:

foreach my $key (keys %extra) {
  $self->{$key} = $extra{$key};
}