#!/usr/bin/perl -w

use strict;
use 5.010;

use Some::Module;

# Set goals
my $cb = Some::Module->new( sub { say 'BOOM!' } );
$cb->begin( qw(foo bar) );

# Much later, as tasks start getting done
$cb->end( foo => 42 );       # "return" value from task 'foo'
$cb->begin( 'baz' );         # can add more tasks, why not
$cb->end( 'bar' );           # just finish task 'bar'
# still waiting for 'baz' to finish at this point

# These shall not pass 
$cb->end( foo => 41 );       # not a second time
$cb->end( food => 'bard' );  # we didn't expect that
# Not fond of dying in async code, so probably 
#   add some customizable error handler,
#   say $cb->on_error(sub {...}); or like that

# Finally, last hanging task is done
$cb->end( baz => 137 );      # BOOM!
# at this point, sub {}->( { foo=>42, bar=>undef, baz=>137 } ) has been called