An additional difference is that if you call a sub with foo; (no parens) you're calling it without arguments.

Not strictly true. Calling foo (or &foo) without arguments will pass on the current subroutine's @_ to the called subroutine.

Update:
Actually, I'm being less than 100% accurate here :)

&foo; will pass on the calling sub's @_, foo; doesn't.

Apologies for any confusion caused.

--
<http://www.dave.org.uk>

"Perl makes the fun jobs fun
and the boring jobs bearable" - me