use Scalar::Util qw(reftype);

tie(%hash, "ScalarHash");
function_that_expects_a_hash_reference(\%hash);

sub function_that_expects_a_hash_reference
{
    my($hash_ref) = @_;
    reftype($hash_ref) eq 'HASH'
        or die "CALLED WITHOUT A HASH REFERENCE!\n";
    ... $hash_ref->{...} ...
}

package ScalarHash;

sub TIEHASH { bless \(my $foo), $_[0] }