an explanation is in the docs, perhaps a little obscured. from perl 5.8.0's perlsub SYNOPSIS:

<snip>To declare subroutines:

    sub NAME;			  # A "forward" declaration.
    sub NAME(PROTO);		  #  ditto, but with prototypes
    sub NAME : ATTRS;		  #  with attributes
    sub NAME(PROTO) : ATTRS;	  #  with attributes and prototypes
    ...

</snip>

and

<snip>to call subroutines:

    NAME LIST;	   # Parentheses optional if predeclared/imported.

</snip>

~Particle *accelerates*