Calculate checksum with Perl

Swedish personal and organization identity number consists of 10 digits and the tenth digit is the checksum. This Perl script calculates checksum for the identity number.

#!/usr/bin/perl

die "checksum.pl identity\n" unless @ARGV > 0 && length $ARGV[0] == 9;

sub checksum
{
  my @pnr = split(//, shift);
  my $n = 2;
  my $sum;

  foreach (@pnr) {
    my $tmp = $_ * $n;
    if($tmp > 9) { $sum += 1 + ($tmp % 10) }
    else { $sum += $tmp; }

    $n = ($n == 2) ? 1:2;
  }

  $tmp = substr($sum, length($sum) -1);
  return ($tmp == 0) ? 0 : (10-$tmp);
}

print $ARGV[0], checksum($ARGV[0]), "\n";