use Smart::Comments;
 ### Expected: 2 * $prediction
 ###      Got: $result
would prints:
 ### Expected: 42
 ###      Got: 13

如果只是perl表达式，则：
### 2 * $prediction
### $result
would prints:
### 2 * $prediction: 42
### $result: 13

调试完毕后，不需要移除这些注释行，只需要注释掉第一行即可。

另外这个模块还提供了processing bar功能：
#### Debugging here...

    for (@values) {         ### Progress: 0...  100
        do_stuff();
    }

Click the url below for more details:
http://search.cpan.org/dist/Smart-Comments-v1.0.3/lib/Smart/Comments.pm

在debug时，如果只是试验，最简单的调试方法就是到处添加一些print()语句。
但是这种方式的弊病是，不容易发现一些特殊的字符输出，比如\n\r\t

范例：
一个常见的错误是当从外部资源读入数据时，忘记使用chomp()。
while (<$file_list>){
  warn "Processing $_";
  next unless -e $_;
  &process_file($_);
}
输出中看不出什么问题，但是&process_file sub 却不会执行。
改变调试行，让文件名更加明显：
while (<$file_list>){
  warn "Processing '$_'";
  next unless -e $_;
  &process_file($_);
}
我们将发现所有的文件名末尾都有换行符，fix it：
while (<$file_list>){
  chomp;
  next unless -e $_;
  &process_file($_);
}

1、使用 -wc 编译：（perl不会运行此程序）
$perl -wc what_went_wrong.pl

2、中分法判断出错代码：
在中间行行头插入 __END__ 标记，再次运行perl -wc。如果出错，则错误在文件前一半，否则后一半。
（标记后的代码都会被转为数据，从而使得Perl忽略它们。）

use Net::FTP;

$ftp = Net::FTP->new("some.host.name", Debug => 0)
  or die "Cannot connect to some.host.name: $@";

$ftp->login("anonymous",'-anonymous@')
  or die "Cannot login ", $ftp->message;

$ftp->cwd("/pub")
  or die "Cannot change working directory ", $ftp->message;

$ftp->get("that.file")
  or die "get failed ", $ftp->message;

$ftp->quit;

use warnings;
use Net::Telnet ();
my ($hostname, $username, $passwd);
$hostname = "192.168.120.48";
$username = "cnshroot";
$passwd = "shroot";
$telnet = new Net::Telnet (Timeout => 10,
                      Prompt => '/-bash-3\.00# $/');
$telnet->open($hostname);
$telnet->login($username, $passwd);
@lines = $telnet->cmd("who");
print @lines;
$telnet->close;
exit;

$logfile = "installation.log";
$endOfFile = "Log_is_the_end_of_file";
$installfile = "interactivelyInstallPanINM.pl";

## Clean up log file
open CLEAN, "> $logfile"
     or die "Can't open the file: $logfile! ($!)";
close CLEAN;

## Call $installfile to write log into the log file
open FH, "interactivelyInstallPanINM.pl |" or die "fork: $!";

## Simulate the tail command.
open FILE, "< $logfile"
     or die "Can't open the file: $logfile! ($!)";

## read until end of file
while (1) {
    my $_ = <FILE>;
    if (/\b$endOfFile\b/){
      last;
    }else{
     print;
    }
}
## last will goto here
close FILE;
close FH;

use Net::SMTP;

my $smtpSer = 'mailhost';
my $from = 'from@xxx.com';
my $to = 'to@xxx.com';
my $subject = 'Test';
my $body = "A simple test message";

$smtp = Net::SMTP->new($smtpSer);

$smtp->mail($ENV{USER});
$smtp->to($to);

$smtp->data();
$smtp->datasend("From: $from\n");
$smtp->datasend("To: $to\n");
$smtp->datasend("Subject: $subject\n");
$smtp->datasend("Body: $body\n");
$smtp->dataend();

$smtp->quit;

##############################################################################

perldoc Net::SMTP for more details

If you need authentication (ie. server does not relay) then substitute Net::SMTP for Net::SMTP_auth. The trick to make this two work together is to create a email message with MIME::Lite (you can add attachments, use html or text) then convert the MIME::Lite object into a string (there is a method to do it within the module) then using the datasend(string) method from Net::SMTP your email will be ready to go.

use warnings;
sub whichIs{
 my ($str, @list) = @_;
 foreach (0..$#list){
  print $list[$_], "\n";
  if ($str eq $list[$_]){
   return $_;
  } 
 }
 -1;
}
my @names = qw / a b c fda wer fda fwefwkjl xxx dfao/;
my $wanna = "xxx";
print "\$wanna - $wanna is the ", &whichIs($wanna, @names), " of @names.\n";

#############################################################################
C:\Perl\learn>"which is.pl"
a
b
c
fda
wer
fda
fwefwkjl
xxx
$wanna - xxx is the 7 of a b c fda wer fda fwefwkjl xxx dfao.

my @items = qw / wilma dino pebbles /;
printf "The items are: \n".("%10s\n" x @items), @items;

########################################################
C:\Perl\learn>printf.pl
The items are:
     wilma
      dino
   pebbles

use warnings;
sub max{
 my $num;
 foreach (@_){
  if ($num < $_){
   #print $num, "\n";
   $num = $_;
  }
 }
 $num;
}
@numbers = qw /342 6436 43 1 -13 55555 431/;
print "The max number is ", &max(@numbers), ".\n";

###################################################
Print log is as below:

C:\Perl\learn>"high-water mark.pl"
Use of uninitialized value $num in numeric lt (<) at C:\Perl\learn\high-water ma
rk.pl line 5.
The max number is 55555.

测试工作中文本处理是一项非常繁琐的事情，利用Perl可以事半功倍。下面的代码将从杂乱的文本文件tcs.TXT中提取测试用例名称，并保存到testcase.txt。详见附件。

use warnings;
my $filename = "tcs.TXT";
my $exportFile = "testcase.txt";
open FILE, $filename
 or die "Can't open $filename: $!";
open TC, ">$exportFile"
 or die "Can't create $exportFile: $!";

while(<FILE>){
 chomp;
 if (/TEST CASE: (.*)     /){
  chomp ($1);
  print TC $1;
 }
 if (/DEVICE UNDER TEST: (.*)\s+/){
  chomp ($1);
  print TC "_$1\n";
 }
}

我们经常需要浏览大量的日志文件，so，将exception or error抓出来是非常有用的。下面的代码将读取服务器上的javlin.log文件，并将含有exception or error关键字的行保存到本地的Err.log文件里。

use warnings;
my $file = '\\\\192.168.127.143\JLogger\javlin.log';
open JLOG, $file
 or die "Can't open $file! ($!)";
if (open ERRLOG, ">Err.log"){
 while (<JLOG>){
  if(/exception|error/i){
   print ERRLOG $_;
  }  
 }
 close ERRLOG;
} else {
 die "Can't create Err.log: $!"; 
}
close JLOG;

print "Hello Perl!\n";