Others pointed out about using the reference directly with the -> dereferencing syntax. That's what I would do.

As of perl 5.22 you can also use refaliasing. In your specific example - where you want to modify the input hashes - you might do as follows:

#!/usr/bin/env perl
use strict;
use warnings;
use Data::Dumper;

use 5.024;
use feature 'refaliasing';
no warnings 'experimental::refaliasing';

my %x = ("a"  => "red");
my %y = ("b"  => "green");
my %z = ("c"  => "black");

modfifyHash(\(%x, %y, %z));

say Dumper \(%x, %y, %z);

sub modfifyHash {
   \my (%x, %y, %z) = @_;
   $x{ "a" } = "circle";
   $y{ "b" } = "square";
   $z{ "c" } = "rectangle";
}

__END__
$VAR1 = {
          'a' => 'circle'
        };
$VAR2 = {
          'b' => 'square'
        };
$VAR3 = {
          'c' => 'rectangle'
        };
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This is probably the closest approximation to pass-by-reference without resorting to prototypes (it only costs you two backslashes in this case).