武钢黄浩东图片:哪位大侠帮偶解释一个perl程序

教材中的例子：哪位大侠帮偶解释一下
22: $lettercount{$a} <=> $lettercount{$b};
是什么意思！是返回关联数组的值吗？

下例从STDIN输入一个文件并输出五个出现频率最高的字母及出现次数。

1 : #!/usr/local/bin/perl
2 :
3 : while ($line = ) {
4 : $line =~ tr/A-Z/a-z/;
5 : $line =~ s/[^a-z]//g;
6 : @letters = split(//, $line);
7 : foreach $letter (@letters) {
8 : $lettercount{$letter} += 1;
9 : }
10: }
11:
12: $~ = "WRITEHEADER";
13: write;
14: $count = 0;
15: foreach $letter (reverse sort occurrences
16: (keys(%lettercount))) {
17: &write_letter($letter, $lettercount{$letter});
18: last if (++$count == 5);
19: }
20:
21: sub occurrences {
22: $lettercount{$a} <=> $lettercount{$b};
23: }
24: sub write_letter {
25: local($letter, $value) = @_;
26:
27: $~ = "WRITELETTER";
28: write;
29: }
30: format WRITEHEADER =
31: The five most frequently occurring letters are:
32: .
33: format WRITELETTER =
34: @: @<<<<<<
35: $letter, $value
36: .

运行结果如下：
$ program
This is a test file.
This test file contains some input.
The quick brown fox jumped over the lazy dog.
^D
The five most frequently occurring letters are:
t: 10
e: 9
i: 8
s: 7
o: 6
还是不太明白：
<=> 是 整数比较操作符 吗？返回 1, 0, or -1
函数occurrences 的返回是什么？
参数 keys(%lettercount) 是怎么带入函数occurrences 中的？