returning refs can be done as

 return \(%x, %y, %z)

but if your goal is to modify these hashes, why don't you operate on the references without returning them?

use strict;
use warnings;
use Data::Dump;

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

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


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

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

dd \(%x, %y, %z);
[download]

now tested!

({ a => "red" }, { b => "green" }, { c => "black" })
({ a => "circle" }, { b => "square" }, { c => "rectangle" })
[download]

update

added tested version