sub NAME BLOCK
    sub NAME (PROTO) BLOCK
    sub NAME : ATTRS BLOCK
    sub NAME (PROTO) : ATTRS BLOCK
            This is subroutine definition, not a real function *per se*.
            Without a BLOCK it's just a forward declaration. Without a NAME,
            it's an anonymous function declaration, so does return a value:
            the CODE ref of the closure just created.

            See perlsub and perlref for details about subroutines and
            references; see attributes and Attribute::Handlers for more
            information about attributes.