Perl

perl -de1

Perl is a declarative, interpreted programming language useful for system administration, CGI programming etc.
Perl is extermely good at string processing. It is not good for things that require efficiency, such as scientific computation or simulations etc.
Perl: Practical Extraction and Report Language.

# -w turns on warning
#!/usr/bin/perl -w

# force declare all variables
use strict;

Perl is case-sensitive. Perl does not require user to declare variables before using them. Every statement must be terminated by a semicolon.
Use my operator to declare variables

my $var;

# must use parantheses
my ($v1, $v2, $v3);

Perl has 3 data types
1. Scalars
2. Arrays
3. Hashes
Scalar variable start with $ sign.
Double-quoted strings are interpolated, any variable name found inside the string will be replaced by their value.
To compare strings, use the string comparison operators: eq (equal), ne (not equal), lt (less than), gt (greater than), le (less or equal), and ge (greater or equal). The string concatenation operator is . (period).
Array variables always start with @ symbol.
Array can contain only scalars, arrays included in other arrays are flattended. While indexing an array, the array should be preceded with $.

my @array = (3, 4, 'five', -0.09876);
print "First element \@array is $array[0].\n"

Negative indices count backward from the end of an array.
Index of last element of @array is given by special scalar $#array;
Hash variables always start with %. A hash is a list of key-value pairs. Hash indexing is done with curly braces.

my %hash = ('name', 'Aamir', 'age', 30, 'occupation', 'engineer');
my %data = (name => 'Aamir', 
    age => 20,
    occupation => 'engineer');

Modern Perl

$ perldoc List::Util
$ perldoc perltoc
$ perldoc Moose::Manual

http://perldoc.perl.org/ http://search.cpan.org/
Perl uses context to express how to treat a piece of data.
The default scalar variable (or topic variable), $_, is most notable in its absence.

chomp $_; # same as chomp;
print;    # prints $_
say;      # prints $_

Perl is sometimes called the Practical Extraction and Report Language
Companies such as ActiveState provide pre-built and enhanced distributions for several platforms, including Windows. You can also get Strawberry Perl for Windows, which comes with all the same stuff as regular Perl plus extra tools to compile and install third-party modules.
ActivePerl from ActiveState has PPM (Perl Package Manager)
Perl doesn’t require any special kind of filename or extension
say (added in v5.10) adds the newline for us

pragmas

use utf8;
use warnings;
perl -w program.pl
#!/usr/bin/perl -w

string operators .: concatenation operator x: repeat operator

A scalar variable holds a single scalar value. Scalar variable names begin with a dollar sign ($), called the sigil, followed by a Perl identifier.
Perl v5.10 adds a slightly better print that it calls say. It automatically puts a newline on the end
When a string literal is double-quoted, it is subject to variable interpolation (besides being checked for backslash escapes)
line-input operator, <STDIN>.
To tell whether a value is undef and not the empty string, use the defined function, which returns false for undef, and true for everything else