It doesn't help that the examples are really boring and kind of obfuscated. For instance: sub promise(&) { $_[0] } That's right, that thing does nothing. I find this really annoying.

It doesn't "do nothing". It constructs a 'promise' from an anonymous block.

That is, it takes an anonymous block of code and returns a code reference to it, so that it can be executed at sometime in the future:

sub promise(&) { $_[0] };;
...
$later = promise{ say 'Keeping my' };;
...
$later->();; ## or just &$later;;
Keeping my
[download]

In other words, it constructs a callback.

To create an 'infinite' stream of values starting at 123 and incrementing by 2:

sub countFromBy{ my( $from, $step ) = @_; return promise{ $from += $st
+ep } };;
...
$counter = countFromBy( 123, 2 );;
...
say &$counter for 1 .. 10;;
125
127
129
131
133
135
137
139
141
143
[download]

Maybe that clarifies things a little for you?