use IO::Select;

# fire off all the ssh requests
my %ssh_handles = ();
foreach my $host ( keys %hosts ) {
    open my $ssh, '-|', "ssh $host 'echo $host'"
        or die "Can't ssh to $host: $!";
    $ssh_handles{$host} = $ssh;
}

# prepare to wait for them to finish
my $select = IO::Select->new();
$select->add( $_ ) for values %ssh_handles;

# wait until they're all done
my @ready;
while ( scalar @ready < scalar keys %ssh_handles ) {
    @ready = $select->can_read();
}

# @ready should be the same as values %ssh_handles
foreach my $host ( keys %hosts ) {
    my $ssh = $ssh_handles{$host};
    my $ssh_result = do { local $/; <$ssh> };
    print "$host said $ssh_result\n";
    close $ssh or warn "Can't close $host ssh: $!";
}