it looks like $ssh is coming in undefined,

That doesn't happen with normal blessed objects:

#! perl -slw
use strict;
use threads;

package Test;

sub new{ bless [], $_[0] }
sub exec{ print 'exec method called from thread: ', threads->tid }

package main;

my $o = Test->new;
$o->exec;

async{
    $o->exec;
}->detach;

sleep 1;

$o->exec;

__END__
c:\test>tobj
exec method called from thread: 0
exec method called from thread: 1
exec method called from thread: 0
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Which probably means that there is some misbegotten code in Net::SSH2 that is attempting to make it "thread-safe".

The next thing to try is creating the ssh object inside the thread:

use threads;
use Net::SSH2;

async {
    my $ssh = Net::SSH2->new( ... );
    $ssh->scp_put($firmware[$antenna_type],'/tmp/fwupdate.bin');
    print "Upgrading firmware, this will take about 3 minutes.\n";
    my $chan = $ssh->channel();
    $chan->exec("/sbin/fwupdate -m\n");
    $chan->close;
}->detach;

...
[download]

Again, that ought to work, but I have no way to test this speculation!