Perl Programming Interview Questions & Answers
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Perl Interview Questions and Answers will guide you that the Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall, a linguist working as a systems administrator for NASA, in 1987, as a general purpose Unix scripting language to make report processing easier. This Perl Interview Questions and Answers Guide will help you to get preparation of job in Perl or learn Pearl by these interview questions and answers.

46 Perl Programming Questions and Answers:

Perl Programming Interview Questions Table of Contents:

Perl Programming Job Interview Questions and Answers
Perl Programming Job Interview Questions and Answers

1 :: How do you give functions private variables that retain their values between calls?

Create a scope surrounding that sub that contains lexicals.
Only lexical variables are truly private, and they will persist even when their block exits if something still cares about them. Thus:
{ my $i = 0; sub next_i { $i++ } sub last_i { --$i } }
creates two functions that share a private variable. The $i variable will not be deallocated when its block goes away because next_i and last_i need to be able to access it.

2 :: How many ways can we express string in Perl?

Many. For example 'this is a string' can be expressed in:
"this is a string"
qq/this is a string like double-quoted string/
qq^this is a string like double-quoted string^
q/this is a string/
q&this is a string&
q(this is a string)

3 :: How do I print the entire contents of an array with Perl?

To answer this question, we first need a sample array. Let's assume that you have an array that contains the name of baseball teams, like this:
@teams = ('cubs', 'reds', 'yankees', 'dodgers');
If you just want to print the array with the array members separated by blank spaces, you can just print the array like this:
@teams = ('cubs', 'reds', 'yankees', 'dodgers');
print "@teamsn";
But that's not usually the case. More often, you want each element printed on a separate line. To achieve this, you can use this code:
@teams = ('cubs', 'reds', 'yankees', 'dodgers');
foreach (@teams) {
print "$_n";
}

4 :: How do you match one letter in the current locale?

/[^W_d]/
We don't have full POSIX regexps, so you can't get at the isalpha() <ctype.h> macro save indirectly. You ask for one byte which is neither a non-alphanumunder, nor an under, nor a numeric. That leaves just the alphas, which is what you want.

5 :: Assume that $ref refers to a scalar, an array, a hash or to some nested data structure. Explain the following statements:

$$ref; # returns a scalar
$$ref[0]; # returns the first element of that array
$ref- > [0]; # returns the first element of that array
@$ref; # returns the contents of that array, or number of elements, in scalar context
$&$ref; # returns the last index in that array
$ref- > [0][5]; # returns the sixth element in the first row
@{$ref- > {key}} # returns the contents of the array that is the value of the key "key"

6 :: What happens to objects lost in "unreachable" memory..... ?

What happens to objects lost in "unreachable" memory, such as the object returned by Ob->new() in `{ my $ap; $ap = [ Ob->new(), $ap ]; }' ?

Their destructors are called when that interpreter thread shuts down.
When the interpreter exits, it first does an exhaustive search looking for anything that it allocated. This allows Perl to be used in embedded and multithreaded applications safely, and furthermore guarantees correctness of object code.

7 :: When would local $_ in a function ruin your day?

When your caller was in the middle for a while(m//g) loop
The /g state on a global variable is not protected by running local on it. That'll teach you to stop using locals. Too bad $_ can't be the target of a my() -- yet.

8 :: How do I read command-line arguments with Perl?

With Perl, command-line arguments are stored in the array named @ARGV.
$ARGV[0] contains the first argument, $ARGV[1] contains the second argument, etc.
$#ARGV is the subscript of the last element of the @ARGV array, so the number of arguments on the command line is $#ARGV + 1.
Here's a simple program:
#!/usr/bin/perl
$numArgs = $#ARGV + 1;
print "thanks, you gave me $numArgs command-line arguments.n";
foreach $argnum (0 .. $#ARGV) {
print "$ARGV[$argnum]n";
}

9 :: How to concatenate strings with Perl?

Method #1 - using Perl's dot operator:
$name = 'checkbook';
$filename = "/tmp/" . $name . ".tmp";

Method #2 - using Perl's join function
$name = "checkbook";
$filename = join "", "/tmp/", $name, ".tmp";

Method #3 - usual way of concatenating strings
$filename = "/tmp/${name}.tmp";

10 :: What is the easiest way to download the contents of a URL with Perl?

Once you have the libwww-perl library, LWP.pm installed, the code is this:
#!/usr/bin/perl
use LWP::Simple;
$url = get 'http://www.websitename.com/';

12 :: How do I do < fill-in-the-blank > for each element in an array?

#!/usr/bin/perl -w
@homeRunHitters = ('McGwire', 'Sosa', 'Maris', 'Ruth');
foreach (@homeRunHitters) {
print "$_ hit a lot of home runs in one yearn";
}

13 :: If EXPR is an arbitrary expression, what is the difference between $Foo::{EXPR} and *{"Foo::".EXPR}?

The second is disallowed under `use strict "refs"'.
Dereferencing a string with *{"STR"} is disallowed under the refs stricture, although *{STR} would not be. This is similar in spirit to the way ${"STR"} is always the symbol table variable, while ${STR} may be the lexical variable. If it's not a bareword, you're playing with the symbol table in a particular dynamic fashion.

14 :: What does length(%HASH) produce if you have thirty-seven random keys in a newly created hash?

5
length() is a built-in prototyped as sub length($), and a scalar prototype silently changes aggregates into radically different forms. The scalar sense of a hash is false (0) if it's empty, otherwise it's a string representing the fullness of the buckets, like "18/32" or "39/64". The length of that string is likely to be 5. Likewise, `length(@a)' would be 2 if there were 37 elements in @a.

15 :: How to dereference a reference?

There are a number of ways to dereference a reference.
Using two dollar signs to dereference a scalar.
$original = $$strref;
Using @ sign to dereference an array.
@list = @$arrayref;
Similar for hashes.

16 :: Does Perl have reference type?

Yes. Perl can make a scalar or hash type reference by using backslash operator.
For example
$str = "here we go"; # a scalar variable
$strref = $str; # a reference to a scalar

@array = (1..10); # an array
$arrayref = @array; # a reference to an array
Note that the reference itself is a scalar.

17 :: What is the difference between /^Foo/s and /^Foo/?

The second would match Foo other than at the start of the record if $* were set.
The deprecated $* flag does double duty, filling the roles of both /s and /m. By using /s, you suppress any settings of that spooky variable, and force your carets and dollars to match only at the ends of the string and not at ends of line as well -- just as they would if $* weren't set at all.

18 :: What value is returned by a lone return; statement?

The undefined value in scalar context, and the empty list value () in list context.
This way functions that wish to return failure can just use a simple return without worrying about the context in which they were called.

20 :: How to read file into hash array ?

open(IN, "<name_file")
or die "Couldn't open file for processing: $!";
while (<IN>) {
chomp;
$hash_table{$_} = 0;
}
close IN;

print "$_ = $hash_table{$_}n" foreach keys %hash_table;

21 :: How do I sort a hash by the hash value?

Here's a program that prints the contents
of the grades hash, sorted numerically by the hash value:

#!/usr/bin/perl -w

# Help sort a hash by the hash 'value', not the 'key'.
to highest).
sub hashValueAscendingNum {
$grades{$a} <=> $grades{$b};
}



# Help sort a hash by the hash 'value', not the 'key'.
# Values are returned in descending numeric order
# (highest to lowest).
sub hashValueDescendingNum {
$grades{$b} <=> $grades{$a};
}


%grades = (
student1 => 90,
student2 => 75,
student3 => 96,
student4 => 55,
student5 => 76,
);

print "ntGRADES IN ASCENDING NUMERIC ORDER:n";
foreach $key (sort hashValueAscendingNum (keys(%grades))) {
print "tt$grades{$key} tt $keyn";
}

print "ntGRADES IN DESCENDING NUMERIC ORDER:n";
foreach $key (sort hashValueDescendingNum (keys(%grades))) {
print "tt$grades{$key} tt $keyn";
}
$cur->new()->{LINK}
The indirect object syntax only has a single token lookahead. That means if new() is a method, it only grabs the very next token, not the entire following expression.
This is why `new $obj[23] arg' does't work, as well as why `print $fh[23] "stuffn"' does't work. Mixing notations between the OO and IO notations is perilous. If you always use arrow syntax for method calls, and nothing else, you'll not be surprised.

23 :: What does read() return at end of file?

0
A defined (but false) 0 value is the proper indication of the end of file for read() and sysread().

24 :: Why does Perl not have overloaded functions?

Because you can inspect the argument count, return context, and object types all by yourself.
In Perl, the number of arguments is trivially available to a function via the scalar sense of @_, the return context via wantarray(), and the types of the arguments via ref() if they're references and simple pattern matching like /^d+$/ otherwise. In languages like C++ where you can't do this, you simply must resort to overloading of functions.

25 :: What does $result = f() .. g() really return?

False so long as f() returns false, after which it returns true until g() returns true, and then starts the cycle again.
This is scalar not list context, so we have the bistable flip-flop range operator famous in parsing of mail messages, as in `$in_body = /^$/ .. eof()'. Except for the first time f() returns true, g() is entirely ignored, and f() will be ignored while g() later when g() is evaluated. Double dot is the inclusive range operator, f() and g() will both be evaluated on the same record. If you don't want that to happen, the exclusive range operator, triple dots, can be used instead. For extra credit, describe this:
$bingo = ( a() .. b() ) ... ( c() .. d() );
Perl Programming Interview Questions and Answers
46 Perl Programming Interview Questions and Answers