## this will undef the array:

  $#array = -1 ; ## but @array = () is the usual.

## To use here-documents with codes, but without
## parse the code, you use '' as a delimiter of the <<EOF:

print <<'CODE';
This $variable wont be parsed,
and this data will br printed as it was.
CODE

## To add some wild data in REGEXP, is common to see
## codes that filter the data before use it in REGEXP,
## but is much more easy to use \Q\E, instead of something
## like =~ s/[\.\\\/]/\\$1/s :

  my $find = 'http://www.domain.foo/' ;
  if ( $_ =~ /\Q$find\E/s ) { ... }

## The use \G to continue the search:

  while( $_ =~ /(.*)(<.*?>)/s ) { ... }
  ## And to get what was left by while (after the last <.*?>):
  my ($final) = ( $data =~ /\G(.*)$/s ) ;

## When we are looking on REGEXP for something that is after other,
## genreally "\n", we forget about the beggin of the string,
## that enables an occurrence that is not after "\n".
## Soo, to do this in only one REGEXP we use (?:^|\n)

  ## remove comments:
  $str =~ s/(?:^|\n)[ \t]*#//gs ;

## Use of substr():

  substr($str , 0 , 0) = ">>" ; ## add data in the begin.
  substr($str , 0 , 1) = "ABC" ; ## replace the 1st char with a bigger text.
  substr($str , -1 , 1) ; ## get the last char.
  substr($str , 0 , -1) ; ## get all the str, but leave the last char out.

## Formatting padding:
  my $var = sprintf("%06d", 123) ;
  print "$var\n" ; ## print 000123

## To generate an array:

  my @lyb = (a..z , A..Z , 0..9) ;
  ## The list:
  ## a b c d e f g h i j k l m n o p q r s t u v w x y z
  ## A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
  ## 0 1 2 3 4 5 6 7 8 9

## Creating an array with 10 elements 'A':
  my @l = ('A') x 10 ;

## creating an HASH with 2 arrays,
## one for the keys and other for the values:
  my %hash ;
  @hash{@keys} = @values ;


## Just to show to beginners the right way for
## concatenation:

$str .= "abc" ; ## and not $str = $str . "abc" ;

## Declaration of my:

  if ( my $foo = <IO> ) { ## $foo is in this scope }

  open(my $fh , ">out") ; ## all in one line
  close($fh);