Ok, there has been a change somewhere along the line: if I compare "n more than ~0" against "~0" for integer n, up to 5.28 it first compares different for n = 2985 (ie with 18446744073709554600); with 5.30 and later it first compares different for n = 2050 (ie with 18446744073709553665):

% /opt/v5.28.1/bin/perl -MMath::BigInt -we '$z0 = Math::BigInt->new(~0
+); for (1..4096) { $z1 = $z0 + $_; $n1 = "$z1"; die $_ if $n1 != ~0 }
+'
2985 at -e line 1.
% /opt/v5.30.0-d/bin/perl -MMath::BigInt -we '$z0 = Math::BigInt->new(
+~0); for (1..4096) { $z1 = $z0 + $_; $n1 = "$z1"; die $_ if $n1 != ~0
+ }'
2050 at -e line 1.
%
[download]

(From the naming I know that's a v5.30.0 built with debugging, but that shouldn't make a difference here.)