$ /usr/bin/perl -v
This is perl, v5.10.0 built for i486-linux-gnu-thread-multi
...

$ perl -e'eval "use threads; async { print qq{ok\n} }->join;"'
ok

##</code><code>##

my $engine;
if ($use_threads) {
   require Engine::Threaded;
   $engine = Engine::Threaded->new();
} else {
   require Engine::Fallback;
   $engine = Engine::Fallback->new();
}