How to force one variable to be an alias to another?

[jdporter]

For example,

my $x;
my $y;
make_alias( $x, $y );
$x = 5;
print $y; # prints 5

Originally posted as a Categorized Question.

[jdporter]

One way is to use the Lexical::Alias module:

use Lexical::Alias;

my( $this, $that );

alias $that, $this;
# $this is now an alias for $that.

Here, $this is made to be an alias of $that. Whatever $this contained prior to the alias call (e.g. the 2 in the above example) is released, much the same as if $this had simply gone out of scope. (That is, its ref count is decremented.) Similarly, the reference count of whatever $that contains is incremented by this operation.

You can also create aliases for array and hash variables this way:

alias @x, @y;
alias %x, %y;

Note that this technique specifically only works for lexical variables; it does not work for package variables or other globals, such as array elements and hash values.

Another, similar approach is to use Tie::Alias. It has the advantage that it's pure Perl, but it's both slower and (currently) only works for scalars.

use Tie::Alias;

my( $this, $that );

tie $this, 'Tie::Alias', \$that;
# $this is now an alias for $that.

[vrk]

Perl 5.22 introduced the refalias feature. The simplest solution to the example is

use v5.22;
use warnings;
use feature 'refalias';

# Silence warnings if desired:
no warnings 'experimental::refaliasing';

my ($x, $y);
\$x = \$y; # alias here
$x = 5;
say $y; # prints 5

[LanX]

Data::Alias is now preferred over Lexical::Alias, as it works with globals (package variables, array elements, hash values, etc.) as well as lexicals.

[ww]

Caution re above: see also [Possible Problems in [id://586121] posted as SOPW (based on 1 line of advice from a "wiser head."

[merlyn]

There are many CPAN modules that have the word Alias in the name, with varying support, features, and documentation. Without further clarification, you'll have to sort them out yourself.

Originally posted as a Categorized Answer.