First, let's just make sure you know there's nothing special about __PACKAGE__ for the purpose of this discussion. Think of it as a function that returns the name of the package in which it resides.

I guess line3 is calling a object method but don't see any new() syntax.

A class method. The LHS of the arrow is a class name ('Foo'), not an object. It's funny you should mention new, since it's usually a class method as well. 

Foo->new            # Calls class method new
Foo->hello          # Calls class method hello
__PACKAGE__->new    # Calls class method new
__PACKAGE__->hello  # Calls class method hello

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Looks like line4 is calling a regular subroutine and that's why the error happens.

Correct. hello expects to be called as a class method or an object method, but it's called as a subroutine. As a result, the arguments it receives are different than those it expects.

Under which kind of situation should we use __PACKAGE__ to create an object instead of calling its new() in the caller program?

Never unless you're told to do so for a particular method

In reply to Re: Question about __PACKAGE__ by ikegami
in thread Question about __PACKAGE__ by sman

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