123  12.4  5E-10  0xff (hex)  0377 (octal)



    'What you $see is (almost) what \n you get'   'Don\'t Walk'



    "How are you?"  "Substitute values of $x and \n in \" quotes."



    `date`   `uptime -u`   `du -sk $filespec | sort -n`



    $x    $list_of_things[5]     $lookup{'key'}

Single-quotes ' ' allow no substitution except for \\ and \'. Double-quotes " " allow substitution of variables like $x and control codes like \n (newline). Back-quotes ` ` also allow substitution, then try to execute the result as a system command, returning as the final value whatever the system command outputs.

    ('Sunday', 'Monday', 'Tuesday', 'Wednesday',

        'Thursday', 'Friday', 'Saturday')



    (13,14,15,16,17,18,19)   equivalent to  (13..19)



    (13,14,15,16,17,18,19)[2..4]  equivalent to  (15,16,17)



    @whole_list

    $DaysInMonth{'January'} = 31;   $enrolled{'Joe College'} = 1;



    $StudentName{654321} = 'Joe College';  



    $score{$studentno,$examno} = 89; 

	

    %whole_hash

Perl 5 allows combinations of these, such as lists of lists and associative arrays of lists.

Name Conventions

Scalar variables start with '$', even when referring to an array element. The variable name reference for a whole list starts with '@', and the variable name reference for a whole associative array starts with '%'.

Lists are indexed with square brackets enclosing a number, normally starting with [0]. In Perl 5, negative subscripts count from the end. Thus, $things[5] is the 6th element of array @things, and

    ('Sun','Mon','Tue','Wed','Thu','Fri','Sat')[1]

Associative arrays are indexed with curly brackets enclosing a string. $whatever, @whatever, and %whatever are three different variables.

    @days = (31,28,31,30,31,30,31,31,30,31,30,31);

                   # A list with 12 elements.



    $#days         # Last index of @days; 11 for above list



    $#days = 7;    # shortens or lengthens list @days to 8 elements



    @days          # ($days[0], $days[1],... )



    @days[3,4,5]   # = (30,31,30)



    @days{'a','c'} # same as ($days{'a'},$days{'c'})



    %days          # (key1, value1, key2, value2, ...)

Case is significant--"$FOO", "$Foo" and "$foo" are all different variables. If a letter or underscore is the first character after the $, @, or %, the rest of the name may also contain digits and underscores. If this character is a digit, the rest must be digits. Perl has several dozen special variables whose second character is non-alphanumeric. For example, $/ is the input record separator, newline "\n" by default. An uninitialized variable has a special "undefined" value which can be detected by the function defined(). Undefined values convert depending on context to 0, null, or false.

The variable "$_" Perl presumes when needed variables are not specified. Thus:

    <STDIN>;          assigns a record from filehandle STDIN to $_

    print;            prints the curent value of $_

    chop;             removes the last character from $_

    @things = split;  parses $_ into white-space delimited

                      words, which become successive

                      elements of list @things.

Subroutines and functions are referenced with an initial '&', which is optional if reference is obviously a subroutine or function such as following the sub, do, and sort directives:

    sub square { return $_[0] ** 2; }



    print "5 squared is ", &square(5);

Filehandles don't start with a special character, and so as to not conflict with reserved words are most reliably specified as uppercase names: INPUT, OUTPUT, STDIN, STDOUT, STDERR, etc.