Thanks that was exactly what I was after! Works great:

use strict;
use warnings;

use AnyEvent;
use AnyEvent::FIFO;

my $cv = AnyEvent::condvar;

my $fifo = AnyEvent::FIFO->new(
    max_active => 1, # max "concurrent" callbacks to execute per slot
);

my @q1 = (9,8,7,6,5,4,3,2,1);
my @q2 = qw(a b c d e f g h i);

foreach my $item (@q1) { $fifo->push( "number_queue", \&callback1, $it
+em); }
foreach my $item (@q2) { $fifo->push( "letter_queue", \&callback2, $it
+em); }

#do other async stuff here...

$cv->recv;

my ($q1t,$q2t);
sub callback1
{
    my ($guard,$item) = @_;
    $q1t = AnyEvent->timer(after=>1, interval => 0, cb => sub {
        print "Got $item\n";
        undef $guard;
    });
}
sub callback2
{
    my ($guard,$item) = @_;
    $q2t = AnyEvent->timer(after=>1, interval => 0, cb => sub {
        print "Got $item\n";
        undef $guard;
    });
}
[download]

Glad it worked out for you!

Why? It's not like you suggested AnyEvent::FIFO. lol