my %defaults = ref( $_[2+$[] ) ? %{$_[2+$[]} : {};

##</code><code>##

sub routine 
{
    my %override = %{$_[$[]};  # reference to a hash.
    my $var      = $_[1+$[];   # scalar
    my %defaults = ref( $_[2+$[] ) ? %{$_[2+$[]} : {}; # optional hash ref.
#blah...

##</code><code>##

prompt> perl -we "print ref( undef() ) ? 'y' : 'n'"
n