In C when you do a malloc(), you are getting a hunk of memory and assigning that address to a variable which points to the memory. Pretty much the same concept in Perl... my $hashref={}; e.g. allocate some memory and assign a reference (pointer) to it to $hashref. There is a kind of weird duality between arrays and hash tables and () would work here. But in general, {} is used for a new anon hash table memory allocation, and [] means a new array memory allocation.

In Perl, there is no need to "define" how you are going to use some memory, you just start using it. A wild concept for a C programmer, but this is true. If some hash key doesn't already exist, like below (username), it will spring automagically into existence! This is called autovivification.

Like in C you can pass a reference (like a pointer) to a sub as is done below. There is no need to prefix a subroutine call with &. You have been reading some truly ancient books! You should buy a new copy of "Programming Perl". There is no need and this is actually a bad idea to attempt to define prototypes for a function (subroutine). Below I call test() before it is "seen by the compiler", this is fine in Perl although that would cause big trouble in C. Of course I would not normally split code around a subroutine, but below it is done to show that it is possible. You can put sub test() anywhere in the file you want.

#!/usr/bin/perl -w
# the above (-w) turns on warnings, Windows ignores the path!!

use strict;
use Data::Dumper;

my $hashref={}; #a reference to anon hash table

$hashref->{username} = 'NOOoooooo!';

print "User Name:  $hashref->{username}\n";

test( $hashref );

sub test  
{
    my $reference = shift;
    my $test = "Pigsy's Perfect Ten";
    $reference->{username} = $test;
    print "Inside Sub Test test\'s value is: $reference->{username}\n"
+;
    $reference->{'accountid'} = '234piggy';
}
print "User Name:  $hashref->{username}\n";
print "Account ID: $hashref->{'accountid'}\n";

__END__
Prints:
User Name:  NOOoooooo!
Inside Sub Test test's value is: Pigsy's Perfect Ten
User Name:  Pigsy's Perfect Ten
Account ID: 234piggy
[download]