1. The  Array  @_

The values passed to a subroutine are stored in the default array @_. 
To get these values, we use, shift function

The expression my ($number1) = shift;  
means: get the first argument from @_ and assign it to $number1

Example:

sub Add1 {
my ($number1) = shift;  
# get first argument from @_ and assign it to $number1
my ($number2) = shift;  
# assign the second argument of @_  to $number2

my $sum = $number1 + $number2;
return $sum;
} 

sub Add2 {
my ($number1,$number2) = @_;
my $sum = $number1 + $number2;
return $sum;
}

sub Add3 {
return ($_[0] + $_[1]);
}


$number1 = 34.00; 
$number2  = 12,12;
$result1 = Add1($number1 ,$number2);
$result2 = Add2($number1 ,$number2);	
$result3 = Add3($number1 ,$number2);	
print " $result1 \n $result2 \n $result3";

outputs 
46
46 
46


2. Conditional Operator
 

(Is something TRUE) ? IF YES do SOMETHING : OTHERWISE other THING ;
The syntax is:
my $RESULT = $BOOLEAN1 ? $VALUE1 : $VALUE2;

Example:
$number1 = -98.6754;
$number2 = ($number1 =~m{^-}) ? abs($number1) : $number1 + 10;
Will ouput 98.6754


3. References
 
Like in C language, A reference points to the variable to which it refers. 
It contains the address of this variable.
In Perl, we can set a reference to a variable by placing a backslash "\" in 
front of this variable:

Example:
my $sentence = 'Best regards .. ';
my $sentence_ref = \$sentence;
\$sentence is the reference of the variable $sentence. 
$\$sentence will return the value of $sentenece, that is: Best regards .. 

4. \Q...\E escape sequence 

All the characters between the \Q and 
the \E are interpreted as literal characters. 


5. Output record separator $/

The default input record separator $/ (or$\) is set to the 
newline \n. To empty is, we will set it as "undifined".

undef $/;

The output record separator is mainly used for the print 
operator "\n".  Default is undef; then set to the newline 
character "\n".

 Perl uses this variable to identify
where to break lines in a file.


For example if it sets a colon (:) to be used as the delimiter
$/ = ".";. Perl will break the file each time it encounters the 
period(.).

Example:
Here is the contents of the file test.txt:

On the desk is a prepositional phrase.  It 
consists of preposition and an 
object of a preposition. 
The object of preposition is a noun.
Astronauts walked on the moon in 1969.
Place is mentioned before time.

Here is the script:
#!c:/Perl/bin
# Per reads file into array
open (FILE, "ajaja.txt");
@lines = ;

# Now, iterate over the file and print each line
foreach $line (@lines)
{
print "- " . $line;
}
# print the related number of lines in the file:
$count = @lines;
print "\n";
print "$count lines in this file!\n";

outputs:
- On the desk is a prepositional phrase.  It
- consists of preposition and an
- object of a preposition.
- The object of preposition is a noun.
- Astronauts walked on the moon in 1969.
- Place is mentioned before time.
6 lines in this file!


with:
undef $/;
outputs:
- On the desk is a prepositional phrase.  It
consists of preposition and an
object of a preposition.
The object of preposition is a noun.
Astronauts walked on the moon in 1969.
Place is mentioned before time.
1 lines in this file!

with:
$/ = "of";
outputs:
- On the desk is a prepositional phrase.  It
consists of-  preposition and an
object of-  a preposition.
The object of-  preposition is a noun.
Astronauts walked on the moon in 1969.
Place is mentioned before time.
4 lines in this file!

6. String Comparisons

$string =~ m/some_text/;

Returns true if the string $string contains a substring 
"some_textt", false otherwise. 


If we need only those strings where "some_text" appears 
at the very beginning, we write:

$string =~ m/^some_text/;

If we want to know whether "some_text" is the very last 
text in the string, we write:

$string =~ m/some_text$/;

Now, if we want the comparison to be true only 
if $string contains nothinng but "some_text" , write:

$string =~ m/^some_text$/;

Now what if we want the comparison to be case insensitive, 
we add the letter i after the ending delimiter:

$string =~ m/^some_text$/i;


Remark:
The ^ operator indicates "beginning of string". 
The $ operator indicates "end of string". 
m: stands for "match"
=~ binding Operator, it binds a scalar expression to 
a pattern match, s, or tr ,...