入门perl程序,该如何处理

入门perl程序
%course=(
  '2CPR2B'=>'C Language',
  '1UNX1B'=>'Intro to UnIX',
  '3SH414'=>'Shell Programming',
  '4PL400'=>'Perl Programming',
  );
sub asc_sort{
  $course{$a} cmp $course{$b};  
}
foreach $key (sort asc_sort(keys(%course))){
  printf "\t% -20s%5d\n", $key, $course{$key};
}
print 'please input course No. 2CPR2B, 1UNX1B, 3SH414, 4PL400',".\n";
chomp($c=<STDIN>);
print "$course{$c}\n";

为什么打印排序后的散列键正确，值全部是0？

[解决办法]

Perl code

  printf "\t% -20s%s\n", $key, $course{$key};
[解决办法]
探讨


为什么呢？大侠能解释下吗？

[解决办法]
%d 是整型
%s 才是字符串