三、在打印格式中显示值
我们使用打印格式的主要原因当然是格式化存贮在简单变量或数组变量中的值从而生成可读性好的输出，这一目的用“值域”来实现。每个值域指定一个值，如变量或表达式，调用write函数时，该值就以值域指定的格式显示。
1、通用的打印格式
打印格式的一个缺点是定义中包含了变量名，例如：
format MYFORMAT =
==========================================================
The winning number is @<<<<< $winnum
==========================================================
.
当调用write输出此格式时，必须记着它使用了变量$winnum。用子程序和局域变量就可以创建更通用的打印格式。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
$