Good observation!

But also note that pragma '_quicksort' is undocumented e.g. in Perl 5.32 (5.32.0/sort). The information about default sorting algorithm seems to be absent in documentation*, sadly :(

Upd. * It is absent in 5.32.0/sort and 5.32.0/functions/sort also, but it is documented in perlsec. I asked about it in an issue and later found that missing info sentence is going to be added into proper place IMO.