Many thanks for providing this wonderful challenge! If all goes at the pace it's rolling now, the fun may well continue into the month of May! Therefore, a warning: some people are at danger of gaining (or loosing) more than what they have bargained for!:)

I decided to ask people to output the top 20, because that presents an interesting mini-challenge by itself

Well said, dear rjt, well said. But it seems it were YOU, who fell into the trap that you so cunningly crafted for poor innocent learners! The top 20 out of million Collatz sequences has an insidious property: there are six "445" lengths in 1e6, but only four in top-20 (so, all six are in top-22). Which to extract? Aha!

Well, the challenge did not clearly state how to order numbers with the same Collatz lengths (CL). But I think it is reasonable to assume, that, since numbers with longer CL are rated better/closer to top, then smaller numbers among producing same CL are to be valued more. Like: "Look! This brave little number commendably creates as long CL as that huge number! And this undeservedly huge number has only managed to generate so puny CL! Loser!"

At least, there must be some consistency in arranging results, don't you agree? In other words, I think that if CL column descends, then numbers column must ascend (for equal CLs). See ordering for CL 450, too.

Well, dear Perl users, I happened to notice this, because in another my (unpublished) solution I had to endure great pain in arranging top-20 properly. Just how many of you, looking at output of (almost all) scripts in this and related 2 threads, have noticed, that top 20 are neatly ordered? And yet, this "natural" result comes at no cost at all, with Perl! Just appreciate what you so ungratefully consume!:)

To illustrate, rjt, here's, in parallel, output of your script and Laurent_R's with marioroy fixes:

Collatz(837799) has sequence length of 525 steps | 837799: 525
Collatz(626331) has sequence length of 509 steps | 626331: 509
Collatz(939497) has sequence length of 507 steps | 939497: 507
Collatz(704623) has sequence length of 504 steps | 704623: 504
Collatz(910107) has sequence length of 476 steps | 910107: 476
Collatz(927003) has sequence length of 476 steps | 927003: 476
Collatz(511935) has sequence length of 470 steps | 511935: 470
Collatz(767903) has sequence length of 468 steps | 767903: 468
Collatz(796095) has sequence length of 468 steps | 796095: 468
Collatz(970599) has sequence length of 458 steps | 970599: 458
Collatz(546681) has sequence length of 452 steps | 546681: 452
Collatz(820022) has sequence length of 450 steps | 818943: 450
Collatz(818943) has sequence length of 450 steps | 820022: 450
Collatz(820023) has sequence length of 450 steps | 820023: 450
Collatz(410011) has sequence length of 449 steps | 410011: 449
Collatz(615017) has sequence length of 447 steps | 615017: 447
Collatz(922526) has sequence length of 445 steps | 886953: 445
Collatz(922526) has sequence length of 445 steps | 906175: 445
Collatz(886953) has sequence length of 445 steps | 922524: 445
Collatz(906175) has sequence length of 445 steps | 922525: 445
[download]

But wait... What's that??? The 922526 number is listed twice, on the left?? Is this... can't believe... is this because of infamous What Every Computer Scientist Should Know About Floating-Point Arithmetic? Because someone:) decided to cut corners and resort to FP? Or is it for another reason? Didn't investigate yet. Just who would have thought that this would surface in so innocent task "calculate lengths of Collatz sequences". Wow! Great challenge!

Edit. No, of course it's not floating point issue. Algo is broken. CLs for odd numbers are cached, but for even numbers they are not. Some even numbers are never passed to &top (e.g. 922524), others are pumped into this subroutine several times. The 922526 hadn't been phased out from @top by odd numbers with longer CLs. With $top large enough, there are many even dupes.