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Perl Interview Questions and Answers

Why should I use the -w argument with my Perl programs?
Many Perl developers use the -w option of the interpreter, especially during the development stages of an application. This warning option turns on many warning messages that can help you understand and debug your applications.
To use this option on Unix systems, just include it on the first line of the program, like this:
#!/usr/bin/perl -w
If you develop Perl apps on a DOS/Windows computer, and you're creating a program named myApp.pl, you can turn on the warning messages when you run your program like this:
perl -w myApp.pl

Assuming $_ contains HTML, which of the following substitutions will remove all tags in it?
1.s/<.*>//g;
2.s/<.*?>//gs;
3.s/<\/?[A-Z]\w*(?:\s+[A-Z]\w*(?:\s*=\s*(?:(["']).*?\1|[\w-.]+))?)*\s*>//gsix;

You can't do that.
If it weren't for HTML comments, improperly formatted HTML, and tags with interesting data like < SCRIPT >, you could do this. Alas, you cannot. It takes a lot more smarts, and quite frankly, a real parser. 

I want users send data by formmail but when they send nothing or call it from web site they will see error.
codes in PHP like this:
if (isset($HTTP_POST_VARS)){
..........
}
else{
echo ("error lalalalal")
}
How it will look in perl?

In php it will be like
if (isset($HTTP_POST_VARS)){
....
}
In perl, tried this.
if ($ENV{'REQUEST_METHOD'} eq 'POST'){
.....
}

What is the output of the following Perl program?
1 $p1 = "prog1.java";
2 $p1 =~ s/(.*)\.java/$1.cpp/;
3 print "$p1\n";

prog1.cpp

Why aren't Perl's patterns regular expressions?

Because Perl patterns have backreferences.
A regular expression by definition must be able to determine the next state in the finite automaton without requiring any extra memory to keep around previous state. A pattern /([ab]+)c\1/ requires the state machine to remember old states, and thus disqualifies such patterns as being regular expressions in the classic sense of the term.

What does Perl do if you try to exploit the execve(2) race involving setuid scripts?
Sends mail to root and exits.
It has been said that all programs advance to the point of being able to automatically read mail. While not quite at that point (well, without having a module loaded), Perl does at least automatically send it.

How do I do < fill-in-the-blank > for each element in a hash?
Here's a simple technique to process each element in a hash:

#!/usr/bin/perl -w

%days = (
'Sun' =>'Sunday',
'Mon' => 'Monday',
'Tue' => 'Tuesday',
'Wed' => 'Wednesday',
'Thu' => 'Thursday',
'Fri' => 'Friday',
'Sat' => 'Saturday' );

foreach $key (sort keys %days) {
print "The long name for $key is $days{$key}.\n";
}

How do I sort a hash by the hash key?
Suppose we have a class of five students.
Their names are kim, al, rocky, chrisy, and jane.

Here's a test program that prints the contents
of the grades hash, sorted by student name:

#!/usr/bin/perl -w

%grades = (
kim => 96,
al => 63,
rocky => 87,
chrisy => 96,
jane => 79,
);

print "\n\tGRADES SORTED BY STUDENT NAME:\n";
foreach $key (sort (keys(%grades))) {
print "\t\t$key \t\t$grades{$key}\n";
}

The output of this program looks like this:

GRADES SORTED BY STUDENT NAME:
al 63
chrisy 96
jane 79
kim 96
rocky 87

}

How do you print out the next line from a filehandle with all its bytes reversed?
print scalar reverse scalar <FH>
Surprisingly enough, you have to put both the reverse and the <FH> into scalar context separately for this to work.

How do I send e-mail from a Perl/CGI program on a Unix system?
Sending e-mail from a Perl/CGI program on a Unix computer system is usually pretty simple. Most Perl programs directly invoke the Unix sendmail program. We'll go through a quick example here.
Assuming that you've already have e-mail information you need, such as the send-to address and subject, you can use these next steps to generate and send the e-mail message:
# the rest of your program is up here ...
open(MAIL, "|/usr/lib/sendmail -t");
print MAIL "To: $sendToAddress\n";
print MAIL "From: $myEmailAddress\n";
print MAIL "Subject: $subject\n";
print MAIL "This is the message body.\n";
print MAIL "Put your message here in the body.\n";
close (MAIL);

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