my $count = 0;
my $fh_out;
while (<$fh_in>) {
   if (!defined($fh_out)) {
      my $fname = sprintf('%s-%d.%s', $base, $count++, $ext);
      open($fh_out, '>', $fname)
         or die("Unable to create \"$fname\": $!\n");
   }

   print $fh_out $_;

   if ($. % $num_lines == 0) {
      undef($fh_out);
   }
}