Perl

It's no surprise that this one pops up often in Perl interview questions and answers.

Yes, Perl supports the concept of objects. But it is not compulsory to use objects. Perl allows using various object-oriented concepts without using or understanding the objects. But in case the developer is creating a heavy application and the program is too large, then it is suitable for the programmer to make it object-oriented.

Yes. Like other programming languages like C, C++, Java, C#, Python, etc., Perl is a case-sensitive programming language.

The acronym CPAN stands for Comprehensive Perl Archive Network. It is a Perl repository that contains 250,000 Perl software development modules. Also, all of these come with accompanying documentation for 39,000 distributions, with around 12,000 contributors. Most of the CPAN modules are free and open source. It acts as an archive network or the Perl program or as a Package manager for the automated software installer.  

The say() function in Perl is like the print() function which displays any string or variable passed within it as arguments. The only difference between say() and print() is that it automatically appends a new line at the end of every output string.

In Perl, hashes are a collection of key-value pairs having unordered forms. In this collection, the keys are unique strings, and their corresponding values are scalar elements. Hashes are preceded with a percent (%) sign. They get indexed and accessed through their key values. 

Scalars are variables that can contain only a single unit of data at a time. That data is said to remain stored in scalar variables. Perl supports different types of scalar values. These are character, string, numbers, floating-point values, references, etc. The $ (dollar) sign is used to create scalars.  

Arrays in Perl contain an ordered list of scalar values. It has a preceding at (@) sign. For accessing a single element from within the Perl array, developers use the dollar ($) sign. The syntax for declaring arrays in Perl is: 

@array_Name = (1stElement, 2ndElement, 3rdElement.... nthElement); 

Yes, Perl programmers can load binary extensions dynamically. But, for that, your system should support more functionality than what is currently available, so it can load it up and keeps running. But then, if the binary extension does not recommend it, programmers have to calculate & compile the extension manually.

One of the most frequently posed Perl interview questions for experienced, be ready for it.  

Perl array contains a set of specific functions associated with the Perl array. These functions help in adding or removing elements to or from an array. There are four different types of Perl array functions. These are: 

• Push: Pushes one element in the array 
• Pop: Pulls out one element from the array 
• Shift: Returns the first value in an array, removing it and shifting the elements of the array list to the left by one. 
• Unshift: Places the given list of elements at the beginning of an array by shifting all the values in the array by right. 

The void context is a type of context that does not care about the return value is. Also, it does not demand a return value when applied within the program. 

The merged array function in Perl helps in merging two arrays into a single array. It eliminates all the commas residing in between them. 

@arrayOne = ("Gaurav", "Karlos", "Ray", "is", "here");   
@arrayTwo = ("He", "wrote", "this");   
# merge operation performed on both the above given arrays 
@merged = (@arrayOne, @arrayTwo);   
print "@merged \n"; 

Programmers use the delete function to remove a hash element from the hash group. It eliminates both keys and their respective value elements from the hash. The syntax of the delete function is: 

delete($hash_name{key_name}); 

Subroutines in Perl are named blocks containing a set of instructions meant for a specific operation. They accept arguments and can return values operated within themselves. Perl programmers can divide their code into meaningful sub-units.  

The syntax is: 

sub subroutine_name { 
   body of the subroutine 
} 
Program: 
# Function definition 
sub Hey { 
   print "Hey, Karlos ! \n"; 
} 
# Function call 
Hey(); 

When a programmer creates a variable with the help of the 'my' operator and it is private in nature, they are called the lexical variables. It stays alive from the spot it got declared till the end, where the block (pair of curly braces) finishes. 

Example: 

my $g; 
my @k; 
my %r;

The conditional statement in Perl helps in making decisions. Based on that decision, a valid Perl statement or set of instructions gets executed. Through conditional statements, a programmer checks a condition or set of conditions. Perl caters to 7 different types of conditional statements. These are: 

• if statement 
• if-else statement 
• If-elsif statement 
• unless statement 
• unless else statement 
• unless...elsif..else 
• switch 

The syntax of if else statement in Perl is: 

if(boolean_expression) { 
   # statement(s) will execute if the given condition is true 
} else { 
   # statement(s) will execute if the given condition is false 
} 

A loop statement allows a programmer to perform a repetitive task or execute a set of statements multiple times in iterations. It is essential when programmers need to execute a block of code several times. Iterative statements get executed sequentially. There are specific ways to change the value so that at every iteration, it can show different values. There are five different types of iterative statements in Perl. These are: 

• while loop 
• until loop 
• for loop 
• foreach loop 
• do-while loop 

There are two different functions Perl developers can use to include a Perl module. 

• Use: The method helps in including only the Perl modules (only for including a .pm type file). The included objects get verified during compilation. We do not have to specify the file extension while performing the import with the use function. The loading of the module is done at compile time. 

The syntax is: 

 use Module_name; 

In case you have a module file as “math.pm”, the code will be: 

use math; 

• Require:  The method helps in including libraries and modules. The included objects get verified during runtime. When using the required function, we have to specify the file extension of the module. Also, the loading of the module is carried out at runtime. The syntax is: 

require modulename.extension; 

In case you have a module file as “math.pm”, the code will be: 

require math.pm; 

There are various benefits that Perl caters to its developers and stays ahead of other programming languages. The situations are: 

• When a developer needs faster execution through their application, Perl will provide you solution for that demand.  
• Perl is comfortable with other web scripting languages like HTML and CSS. 
• Perl also caters to a lot of flexibilities when developing web-based applications. 
• Applications that require frequent use of regular expressions and text manipulation can render Perl for their development purpose. 

Apart from all these situations, Perl is free, plus it has a repository of pre-written codes with community support called CPAN (Comprehensive Perl Archive Network). The Comprehensive Perl Archive Network (CPAN) is a repository of over 250,000 software modules. Some of these are: 

• DBI 
• JSON 
• LWP::UserAgent 
• DateTime 
• XML::Simple 
• Text::CSV 
• Spreadsheet::WriteExcel 
• DBD::mysql 
• XML::Parser 
• WWW::Mechanize 
• Excel::Writer::XLSX 
• Net::SSH::Perl 
• Net::SMTP 
• Net::Telnet 
• DBD::Oracle 
• Spreadsheet::ParseExcel 
• IO::Socket::SSL 
• HTTP::Response 
• DBIx::Class::ResultSet 
• Log::Log4perl 

Variables declared using the 'my' keyword reside only within that block scope in which they get declared. Such variables do not get inherited, and the functions do not get visibility when called in that block. Again, when the variables defined as 'local' remain visible in that block. They have visibility in functions when called from that particular block.

In Perl, both Terms and lists have the highest precedence. Terms include expressions in parenthesis, variables, etc. List operators also render the same level of precedence as that of terms. Both these operators have strong left word precedence. 

The entire list of operator precedence in Perl looks like: 

Perl is a feature-rich programming language that can easily handle dates and times within the program. Perl provides a predefined module called the DateTime module. This DateTime is a class that helps in representing various combinations of dates and times. The DateTime module in Perl renders the New Style of the calendar by extending backward in time before its creation (in 1582). This is usually called the “proleptic Gregorian calendar”. In such a calendar, the first day of the calendar (which is also called the epoch) is the first day of year 1 and is believed to be the birth of Jesus Christ.

The syntax is: use DateTime;

Program:

$datetime = localtime();   
print "Local Time of the System : $datetime\n"; 

Perl's goto statement is a jumping statement, that makes a jump from one part of the program to the other when the program encounters the goto label. It is sometimes referred to as the unconditional jump statement because the jumping from one section of the program to the other takes place without any condition. Programmers can use the goto statement anywhere within a function. 

The syntax is:  

LABEL_NAME: 
Statement 1; 
Statement 2; 
. 
. 
. 
. 
. 
Statement n; 
goto LABEL_NAME; 
 
Program: 
sub dispNumb()  
{  
    my $numb = 1;  
lab:  
    print "$numb ";  
    $numb++;  
    if ($numb <= 20)  
    { 
        goto lab; 
    } 
}  
     
# Driver Code 
dispNumb(); 

Escape sequences are special characters that do not represent themselves when used as a part of string or character literal. All these characters are preceded by a backslash.

For example: ‘\n’, ‘\t’, ‘\\’, etc.

Program:

$result = "This is Mr. \" Karlos \""; 
print "$result \t"; 
print "\$result \n";

The redo statement in Perl restarts the existing loop without evaluating the control statement. Also, further statements within the block won't get executed. 

Program: 

$numb = 10;   
while($numb < 150){   
   if( $numb == 50 ){   
     $numb = $a + 10;   
      redo;   
   }   
   print "Number = $numb \n";   
} continue {   
   $numb = $numb * 2;   
}