When should I use the & to call a Perl subroutine?

I have heard that people shouldn't be using & to call Perl subs, i.e:

function($a,$b,...);
# opposed to
&function($a,$b,...);

I know for one the argument list becomes optional, but what are some cases where it is appropriate to use the & and the cases where you should absolutely not be using it?

Also how does the performace increase come into play here when omitting the &?