This constructor is written in a confusing way, so you're right to be confused about it. About your questions:

1. The comma operator combines expressions, so shift, shift could just be written as (shift,shift), which is the same as "take the first two elements off of @_". This should have better been written as splice @_, 0, 2.
2. chromatic will wail and rail against ref($class)||$class, but it is a well-entrenched idiom to allow construction of a fresh object from an old object. ref($class) returns the package name of an object or undef if $class is not a reference. This is to make both of these invocations work:

       my $obj = My::Class->new(...);
       my $other_obj = $obj->new(...);
   [download]

   Whether or how that can make sense has been discussed here and elsewhere. In most cases, you can just leave it out.
3. The first parameter of new() is the class name, but it's not hidden, it's there in plain sight:

   My::Class->new("a","b");
   [download]

   is the same as

   My::Class::new("My::Class","a","b");
   [download]

   except that inheritance is not respected in the second case.

4. The while loop is taking the first two elements in @_ and putting them into the hash referenced by $self. Then it removes the first two elements of @_. This loop could have been written more succinct and clear as:

   $self = { @_ };
   # or
   %$self = @_;
   [download]

In short, the whole constructor could have been written as:

sub new {
    my ($class,%args) = @_;
    my $self = \%args;
    bless $self, ref($class)||$class;
};
[download]

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