NAME(LIST);      # & is optional with parentheses.
NAME LIST;       # Parentheses optional if predeclared/imported.
&NAME(LIST);     # Circumvent prototypes.
&NAME;           # Makes current @_ visible to called subroutine.

defined(&NAME);  # & is needed. The same is valid for undef(&NAME) and any case when you are naming your sub
&$subref();      # & is needed. But $subref->() can be used to avoid the &
&{$subref}();    # & is needed. But $subref->() can be used to avoid the &