With OO, the subroutine "meth" could be inherited (exist in another namespace from which the object inherits). When called as a subroutine, the subroutine must exist in the current namespace (or some pretty heavy AUTOLOAD magic should be involved).
Liz
Comment on Re: Re: Re: Re: perl OO - to use or not to use
Other important difference is that in case of OO syntax caller in general doesn't know how $obj will handle method call (i.e. caller doesn't know which subroutine in which package is going to be called) as it depends on the class of $obj.