I had a very brief look at that — I haven't read this previously.

It indicates that code is "syntactic sugar". It's a subroutine definition (see perlsub) with a prototype. It's not actually intended to do anything by itself anymore than any other subroutine definition is.

Following the point where it's introduced, it appears in a number of examples as function arguments. The first one is this:

node($m, promise { upto($m+1, $n) } );
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Here's a very trivial example of usage which, with the information from the links I provided above, may help your understanding.

#!/usr/bin/env perl

use strict;
use warnings;

sub promise (&) { $_[0] }

make_good_on_promise(promise { print @_ }, "Hello.\n");

sub make_good_on_promise {
    my ($promise_to_do, $whatever) = @_;

    $promise_to_do->($whatever);
}
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Sample run (as stated — a trivial example):

$ pm_example.pl
Hello.
$
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