Thank you, that was an insightful reply
First time I tried to shuffle, I used a different Perl module, but exact same syntax otherwise. That module is
use Array::Shuffle qw(shuffle_array shuffle_huge_array); # from https://metacpan.org/source/SALVA/Array-Shuffle-0.03/samples/benchmark.pl
I got inexplicable results using that module to randomly shuffle the sequences. That one is supposed to use the Fisher Yates Knuth algorithm. I contacted the author of the Perl module, and he suggested I cross verify using another Perl module. So using List::Util now.