Pure Perl has neither an explicit alias operator

You can (ab)use typeglobs for this, at least for package variables. For instance:

#!/usr/bin/perl

no strict;
use warnings;
use feature qw/say/;

$a = 42;

*b = *a;
say $b; # 42

$b = 69;
say $a; # 69
[download]

I can't think of a way of doing this with lexicals off the top of my head, though.

nor an explicit unalias operator.²

So if a scalar variable $a is an alias to $b you can't easily say $a=42 without changing $b , i.e. replacing the alias in the $a -"slot" with a literal 42.¹

I don't think this is needed. Suppose that you have variables - let's say lexicals - $a and $b, with the latter being aliased to the former. Unalias $a would mean creating a new lexical that's not related to $b anymore, so why not just declare a new lexical with the same value (my $c = $b) and use that instead? The result's the same, and it's arguably clearer, since if you look at any line in isolation there's no confusion over whether $a is (still) aliased to $b anymore.

OTOH there's something to be said in favor of explicit alias and unalias operators as well. I'd not be surprised at all if there were CPAN modules that implemented this.