$ perl -wMstrict -MData::Dumper -le 'my (%x,%y) = (1,2,3,4); print Dumper(\%x,\%y)'
$VAR1 = {
          '1' => 2,
          '3' => 4
        };
$VAR2 = {};

##</code><code>##

use warnings;
use strict;
use Data::Dump 'pp';

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

pp \%x, \%y, \%z;
my ($r1, $r2, $r3) = modfifyHash(\%x, \%y, \%z);
pp $r1, $r2, $r3;
pp \%x, \%y, \%z; # these remain unmodified

sub modfifyHash {
	#my ($x, $y, $z) = @_; # this would modify the original hashes
    my ($x, $y, $z) = map { { %{ $_ } } } @_;
    #                       ^ ^  ^
    #                       | |  for each argument
    #                       | dereference as hash
    #                       make new hash reference (shallow copy!)
    $x->{a} = "circle";
    $y->{b} = "square";
    $z->{c} = "rectangle";
    return $x, $y, $z;
}

__END__

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