Regarding the time difference when running with and without "use threads", I have discovered that avg() is the culprit.

Hm-m, it's not what I'm observing here. Setting $m = 30_000 and replacing avg with at(0), these are results of 3 runs without thread and in a thread:

14.9562268257141
14.9582891464233
14.8853561878204

11.9686307907104
12.0527169704437
12.0850310325623
[download]

And then replacing all 3 lines of loop block with just simple

my $y = $x-> index( sequence $n )-> at( 0 );
[download]

7.74871516227722
7.80155396461487
7.71721601486206

4.92977499961853
4.87044596672058
4.87968802452087
[download]

So I'd say it's something strange with index going on. I'm not appealing to anyone for investigation :), observable speed difference may depend very much on hardware, let it be a murky PDL mystery.

And another entertaining (but dangerous, maybe) bit of PDL trivia: above I sketched a parallelization solution with threads and random. Except, if there were more than 1 worker thread, it won't work as expected. The random documentation says Perl's srand can be used to seed, and one may assume that random relies on Perl's RNG, including it (automatically) seeds in each thread that's started. Consider:

use strict;
use warnings;
use feature 'say';
use threads;
use PDL;

PDL::no_clone_skip_warning;

srand;
async sub{ say random( 1 )-> at( 0 )}
    for 1 .. 5;
$_-> join for threads-> list;

say rand;

__END__

0.851411183904023
0.851411183904023
0.851411183904023
0.851411183904023
0.851411183904023
0.851411183904023
[download]

That's some randomness. Try to say rand instead. So, one must to explicitly call srand at the start of a thread, if using threads and PDL's random.