What exactly is the difference between an alias and a reference?

• An alias is another name that refers to the same thing as the first.

  In your first example, $_ is another name that can be used to refer to each of the elements $arr[0], $arr[1] etc. in turn.

• A reference, is the "address" of the thing it references.

  If your second example, $aref refers to the entire array. To get to individual elements of that array you must dereference it and index it. eg. $aref->[0], $aref->[1] etc.

how can I create an alias explicitly (without using any extra module).

You can do this using typeglobs.

Eg.

#! perl -slw
use strict;

## An array with some contents
my @array = 'a'..'z';

## Tell strict we are going to use a global
our @alias;

## Set the array reference of the typeglob *alias
## to point to the lexical array we set up earlier
*alias = \@array;

## Now when we use @alias, we are (also) using @array
print $alias[ 0 ], ' : ', $alias[ 25 ];

## changes to either affect both
## Or rather change via either name 
## affect the same storage
$alias[ 0 ] = 'fred';
$array[ 25 ] = 'bill';

print $alias[ 0 ], ' : ', $alias[ 25 ];
print $array[ 0 ], ' : ', $array[ 25 ];


__END__
C:\test>junk
a : z
fred : bill
fred : bill
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