off-by-one error

It seems you've hit some precision limit - which is avoided if you use bigint.
Try placing an extra number at the beginning of $x, and you'll soon see what I mean.
For every extra number you prepend to the start of $x, you get an extra "0" at the end of $y:

use strict;
use warnings;
use feature 'say';

use bignum;#num;
#  (1) #use bignum downgrade => "Math::BigInt", upgrade => "Math::BigI
+nt";
## (2) #use bigint;

say $^V;
my $x = 999999999921778071482940061661655974875633165533182;
say $x;
my $y = $x / 2;
say $y;

__END__

For me, outputs:
D:\pscrpt>perl try.pl
v5.39.5
999999999921778071482940061661655974875633165533182
499999999960889035741470030830827987437800000000000
[download]

Cheers,
Rob

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