%a = qw(cat dog);
print $a{cat};
foo(%a);
print $a{cat};

sub foo { $_[1] = "elephant" }

##</code><code>##

%a = qw(cat dog);
print $a{cat};
%a = foo(%a);       # <-- note assignment
print $a{cat};

sub foo {
    my %temphash = @_;
    $temphash{cat} = "elephant";
    %temphash;
}