Integer has an upper limit. Perl automatically converts to float if integer cannot be used. I was able to get the desired output with bigint:

#!/usr/bin/perl
use warnings;
use strict;

my ($n, $m, $i) = split ' ', <>;
my $ma = $m / $i;
my $na = $n / $i;
$ma = int($ma) + 1 if int $ma != $ma;
$na = int($na) + 1 if int $na != $na;

use bigint;
my $x = int($ma * $na);
print 0 + $x, "\n";
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