Thanks for the clarifications - that helps a lot. Having played around with this now I think that the problem you are having essentially boils down to the fact that the event loop is blocking because you are telling it to and that there's no reason for you to do so.

Firstly, in order to simplify things I have decided to concentrate solely on AnyEvent (because that's what I'm less unfamiliar with) and ditch IO::Async for this illustration. That gets rid of the need for the glue and stops the chances of 2 different event loops coming to blows. As a starting point, here are two timers running on 5 and 7 second intervals implemented purely with AnyEvent. There's a third timer there just so the thing doesn't carry on forever.

#!/usr/bin/env perl 
use strict;
use warnings;

use AnyEvent;

my $cv     = AnyEvent->condvar;
my @timers = (
    AnyEvent->timer (
        after    => 2,
        interval => 5,
        cb       => sub {
            warn "Timer 1 at " . AnyEvent->now . "\n";
        }
    ),      
    AnyEvent->timer (
        after    => 2,
        interval => 7,
        cb       => sub {
            warn "Timer 2 at " . AnyEvent->now . "\n";
        }
    ),  
    AnyEvent->timer (
        after => 20,
        cb    => sub {
            warn "Timer 3 at " . AnyEvent->now . "\n";
            $cv->send;
        }
    ),  
);          
$cv->recv;
[download]

If you run this you will see something along these lines:

Timer 1 at 1591953440.37264
Timer 2 at 1591953440.37264
Timer 1 at 1591953445.37213
Timer 2 at 1591953447.37533
Timer 1 at 1591953450.36854
Timer 2 at 1591953454.38475
Timer 1 at 1591953455.36656
Timer 3 at 1591953458.37977
[download]

So the timers seem to be working on their correct schedules. Now we can add in some pings. I've reduced the numbers here so we don't get bucketloads of output, but the principle should be the same.

#!/usr/bin/env perl 
use strict;
use warnings;

use AnyEvent;
use AnyEvent::Ping;
use Data::Dumper;

my @todoList = map { "192.168.100." . $_ } (1 .. 5);
        
my $cv     = AnyEvent->condvar;
my @timers = (   
    AnyEvent->timer (
        after    => 2,
        interval => 5,
        cb       => sub {
            warn "Timer 1 at " . AnyEvent->now . "\n";
            ping_hosts (1, @todoList);
        }
    ),      
    AnyEvent->timer (
        after    => 2,
        interval => 7,
        cb       => sub {
            warn "Timer 2 at " . AnyEvent->now . "\n";
            ping_hosts (2, @todoList);
        }   
    ),  
    AnyEvent->timer (
        after => 20,
        cb    => sub {
            warn "Timer 3 at " . AnyEvent->now . "\n";
            $cv->send;
        }
    ),
);
$cv->recv;

sub ping_hosts {
    my ($num, @hosts) = @_;

    my $ping_timeout  = 2;     #seconds to wait for response
    my $ping_attempts = 4;     #number of pings to issue,
    my $ping_pktsize  = 32;    #packet size of ping
    my $ping = AnyEvent::Ping->new (packet_size => $ping_pktsize);
    $ping->timeout ($ping_timeout);

    #$cv_ping->begin(sub { shift->send($_[0]) });
    foreach my $host (@hosts) {
        $ping->ping (
            $host,
            $ping_attempts,
            sub {
                print "Pinger $num: $host state:" . Dumper ($_[0]) . "
+\n";
            }
        );
    }
}
[download]

I've simplified your ping loop/sub a bit and you'll notice that there is now no counter and importantly no cond_var. This is because you have no need to block here if all you are doing is firing off the pings. Now when we run this it runs to completion with the pings sent on the intended schedule.

This is my first go at using event loops so I am slowly grasping the concepts and how to implement it, but i'm struggling a little, so I'm not even sure if i'm doing this properly.

The concepts are tough - there's no doubt about that. I don't do much functional programming and even less on event loops so it was interesting to approach this from an equally inexpert standpoint. My simplistic approach with these is just not to block unless it's really necessary. That's not always easy to do when you are coming from a procedural background but it has helped me through when tackling this sort of task.

I hope this is useful for you and wish you luck with the rest of your project.

Not sure if my reply got posted properly.

Thanks again for your help

I did try something similar to this, but the problem I have is that I need the ping_hosts() sub to complete before the timer executes again, essentially blocking the timer. Thats why I had left $cv->recv; inside the ping_hosts() sub. Sorry I may have not made this clear.

From my basic understanding of event loops, this approach isn't going to work as there could be n+1 schedules blocking, I think I need to do something like whats described in http://www.perladvent.org/2014/2014-12-24.html replacing $cv->recv; with $cv->cb($callback); I'll keep plugging away but any assistance is much appreciated!