The & prefix unambiguously means "this is a user-defined subroutine call coming up next". You can omit it when you are using parens after your function name and your function name doesn't conflict with a built-in, or if you've declared/defined your function before you use it and the name doesn't conflict with a built-in.

The & prefix also disables prototype checking, but you shouldn't be using prototypes in the first place, so I consider that a very minor point, but if I didn't mention it, someone else would surely raise that flag. {grin}

In our Learning Perl book, we suggest that you always use the ampersand when you're first learning Perl, because otherwise you'll be a bit befuddled when the following code doesn't work:

sub log {
  print STDERR "logging: ", @_, "\n";
}

...
log("starting");
...
log("finishing");
...
[download]

Yes, you're not calling your log subroutine... you're calling the built-in "logarithm" function. Oops.