$ time perl collatz3_a.pl 1e7

AMD 3970x, Docker Container, Ubuntu + Perlbrew Perl 5.30.1

collatz3_a.pl 1e7  13.130s  (a) original, accepts argument
collatz3_b.pl 1e7  12.394s  (b) a + replaced division with >> 1
collatz3_c.pl 1e7  12.261s  (c) b + removed 1 level of branching
collatz3_d.pl 1e7   9.170s  (d) c + reduced loop iterations
collatz3_e.pl 1e7   7.681s  (e) d + less caching

collatz3_filemap    8.889s    1 core

collatz3_parallel   8.982s    1 core
collatz3_parallel   4.548s    2 cores
collatz3_parallel   2.378s    4 cores
collatz3_parallel   1.233s    8 cores
collatz3_parallel   0.661s   16 cores
collatz3_parallel   0.408s   32 cores

Collatz(8400511) has sequence length of 686 steps
Collatz(8865705) has sequence length of 668 steps
Collatz(6649279) has sequence length of 665 steps
Collatz(9973919) has sequence length of 663 steps
Collatz(6674175) has sequence length of 621 steps
Collatz(7332399) has sequence length of 616 steps
Collatz(7532665) has sequence length of 616 steps
Collatz(5649499) has sequence length of 613 steps
Collatz(8474249) has sequence length of 611 steps
Collatz(6355687) has sequence length of 608 steps
Collatz(8847225) has sequence length of 606 steps
Collatz(9533531) has sequence length of 606 steps
Collatz(6635419) has sequence length of 603 steps
Collatz(9953129) has sequence length of 601 steps
Collatz(7464846) has sequence length of 598 steps
Collatz(7464847) has sequence length of 598 steps
Collatz(3732423) has sequence length of 597 steps
Collatz(5598635) has sequence length of 595 steps
Collatz(8397953) has sequence length of 593 steps
Collatz(6298465) has sequence length of 590 steps