I'd need a way of sharing the Control::CLI (i.e. Net::SSH2) object instead of having it copied to the children (threads::shared does not appear to support the Net::SSH2/Control::CLI object). I've also read here that it may be an SSH security violation to have the parent and its child thread both access the same session. If someone knows a way to do this with ithreads, I'd love to understand how it can be done. :)

Try:

use threads;
use threads::shared qw[ shared_clone ];

my @sessionData = ...;

my @ssh2_handles :shared;

sub opener {
    my( $idx, ... ) = @_;
    use Net::SSH2;
    
    my $ssh = Net::SSH2->new( .. );
    lock @ssh2_handles;
    $ssh2_handles[ $idx ] = shared_clone( $ssh );
    return;
}

my $idx = 0;
my @openers = async( \&opener, $idx++, $_ ) for @sessionData;

$_->join for @openers;

### use @ssh2_handles here...
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This is untested, untried, speculation -- I don't have facilities to do so -- but the basic premise is sound.