The way I always explain it: CODE objects are declared as subroutines, but used either as functions or methods. Or even both, if you want to confuse people (CGI.pm does, apparantly):

sub foo { print @_ }

foo "Hello "; # function - prints "Hello "

main->foo("!"); # method - prints "main!"
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You cannot call a subroutine except as a function or a method.

Nor can you write a function or method, except as a subroutine.