use common;

my $result=internal_days("20080911","20021011");

So far, we've used the method arrow syntax:
Class->method(@args);
or the equivalent:
my $beast = 'Class';
$beast->method(@args);
which constructs an argument list of:
('Class', @args)
and attempts to invoke:
Class::method('Class', @args);


my $tv_horst = Horse->new;
my $noise = $tv_horse->sound;
Now for the fun part: Perl takes the class in which the instance was blessed, in this case, Horse, and uses it to locate and invoke the method, as if we had said Horse->sound instead of $tv_horse->sound. The purpose of the original blessing is to associate a class with that reference to allow Perl to find the proper method.
In this case, Perl finds Horse::sound directly (without using inheritance), yielding the final subroutine invocation:
Horse::sound($tv_horse)
Note that the first parameter here is still the instance, not the name of the class as before. neigh is the return value, which ends up as the earlier $noise variable.

We mentioned that there are two styles of method invocation. The first style for invoking a method looks like this:

INVOCANT->METHOD(LIST)
INVOCANT->METHOD


For obvious reasons, this style is usually called the arrow form of invocation. (Do not confuse -> with =>, the "double-barreled" arrow used as a fancy comma.) Parentheses are required if there are any arguments. When executed, the invocation first locates the subroutine determined jointly by the class of the INVOCANT and the METHOD name, and then calls that subroutine, passing INVOCANT as its first argument.