use Fcntl;   # For O_RDWR, O_CREAT, etc.
use SDBM_File;

# 0666 reduced by umask.
tie my%__intfactor, 'SDBM_File','/var/tmp/intfactor.sdbm',
      O_RDWR|O_CREAT, 0666)
   or { warn"Couldn't tie SDBM file '/var/tmp/intfactor.sdbm': $!";
        warn"continuing with ramdb";
};
sub intfactor() {
  exists $__intfactor{%r} and return $__intfactor{%r};
  $c -= 4096 while $c % $r; 
  return $__intfactor{%r}=$c;
}

# I'm lost by the meaning of your code, where is $s used?
$r = 12; 
$s = 2*1024**3; 

print intfactor($c,$r);