Math::BigInt code reads much more simply if you treat them as numbers; you can also take advantage of the fact that when one of the operands of a numeric operator is a Math::BigInt the other is automatically upgraded:

use Math::Bigint;
my $KB = Math::BigInt->new(1024);
my $MB = $KB * $KB;
my $GB = $MB * $KB;

my $required = shift(@ARGV) * $GB;
my $CDsize = shift(@ARGV) * $MB;
print +($required + $CDsize - 1) / $CDsize;
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Hugo